Russia, the United States and China have all built new facilities and dug new tunnels at their nuclear test sites in recent years, satellite images obtained exclusively by CNN show, at a time when tensions between the three major nuclear powers have risen to their highest in decades.

While there is no evidence to suggest that Russia, the US or China is preparing for an imminent nuclear test, the images, obtained and provided by a prominent analyst in military nonproliferation studies, illustrate recent expansions at three nuclear test sites compared with just a few years ago.

One is operated by China in the Xinjiang Uyghur Autonomous Region, one by Russia in an Arctic Ocean archipelago, and another in the US in the Nevada desert.

The satellite images from the past three to five years show new tunnels under mountains, new roads and storage facilities, as well as increased vehicle traffic coming in and out of the sites, said Jeffrey Lewis, an adjunct professor at the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies.

"There are really a lot of hints that we're seeing that suggest Russia, China and the United States might resume nuclear testing," he said, something none of those countries have done since underground nuclear testing was banned by the 1996 Comprehensive Nuclear Test Ban Treaty. China and the US signed the treaty, but they haven't ratified it.

Retired US Air Force Col. Cedric Leighton, a former intelligence analyst, reviewed the images of the three powers' nuclear sites and came to a similar conclusion.

"It's very clear that all three countries, Russia, China and the United States have invested a great deal of time, effort and money in not only modernizing their nuclear arsenals, but also in preparing the types of activities that would be required for a test," he said.

Moscow has ratified the treaty, but Russian President Vladimir Putin said in February he would order a test, if the US moves first, adding that "no one should have dangerous illusions that global strategic parity can be destroyed."

The expansions risk sparking a race to modernize nuclear weapons testing infrastructure at a time of deep mistrust between Washington and the two authoritarian governments, analysts said, though the idea of actual armed conflict is not considered imminent.

"The threat from nuclear testing is from the degree to which it accelerates the growing arms race between the United States on one hand, and Russia and China on the other," Lewis said. "The consequences of that are that we spend vast sums of money, even though we don't get any safer."

Nuclear threats

Lewis' comments came after a prominent nuclear watchdog group, the Bulletin of the Atomic Scientists, earlier this year set its iconic Doomsday Clock, a measure of how close the world is to self-destruction, to 90 seconds to midnight, the clock's most precarious setting since its inception in 1947.

The group cited the war in Ukraine, sparked by Russia's illegal invasion of its neighbor in February 2022, as main reason for its sobering assessment.

"Russia's thinly veiled threats to use nuclear weapons remind the world that escalation of the conflict -- by accident, intention, or miscalculation -- is a terrible risk. The possibility that the conflict could spin out of anyone's control remains high," the group said.

In other words, the Doomsday Clock today signals a higher risk of the end of humankind than in 1953, when both the United States and the Soviet Union conducted dramatic above-ground tests of nuclear weapons.

Last month United Secretary-General António Guterres issued a fresh appeal for key countries to ratify the international treaty that bans experiments for both peaceful and military purposes

"This year, we face an alarming rise in global mistrust and division," Guterres said. "At a time in which nearly 13,000 nuclear weapons are stockpiled around the world — and countries are working to improve their accuracy, reach and destructive power — this is a recipe for annihilation."

Lewis pointed out that the unexpectedly poor performance of the Russian military in Ukraine could be part of the impetus for Moscow to consider resuming nuclear tests.

Dmitry Medvedev, a hawkish backer of Putin and the current deputy chairman of Russia's Security Council, has vowed Moscow "would have to use nuclear weapons" if the Ukraine counteroffensive became successful. Medvedev's bellicose rhetoric has raised eyebrows, but Putin is Russia's key decision-maker, and widely seen as the real power behind the throne during Medvedev's four-year presidency.

Belarus, which has played a key role in Russia's invasion of Ukraine, has also received tactical nuclear weapons from Moscow, President Alexander Lukashenko said in August. He added that Minsk would be willing to use them in the face of foreign "aggression."

Russia and China

Even as the Russian military was invading Ukraine last year, analysts have also seen an expansion of the country's nuclear test site in Novaya Zemlya in the Arctic Ocean archipelago.

In mid-August, the facility received renewed focus when Defense Minister Sergei Shoigu paid a visit, according to the Russian Defense Ministry.

The Novaya Zemlya site was first used by the Soviet Union to conduct nuclear tests in 1955 until the USSR's final underground explosion in 1990. During that time, the site saw a total of 130 tests involving more than 200 devices, according to a review published in the Science and Global Security journal.

Satellite images obtained by CNN showed that there has been extensive construction at the Novaya Zemlya test site from 2021 to 2023, with ships and new shipping containers arriving at its port, roads being kept clear in the winter, and tunnels dug deep into the Arctic mountains.

"The Russian test site is now open year round, we see them clearing snow off roads, we see them building new facilities." Lewis said.

Near those facilities are tunnels where Russia has tested in past, Lewis said. "In the past five or six years, we've seen Russia dig new tunnels, which suggests that they are prepared to resume nuclear testing," he added.

"It's pretty clear to me that the Russians are gearing up for a possible nuclear test," added Leighton, the former US Air Force intelligenc officer and now a CNN analyst. But he offered what he said were important "caveats."

"The Russians may be trying to go right up to the line by making all the preparations for a nuclear test, but not actually carrying one out. In essence, they'd be doing this to 'scare' the West," Leighton said.

Moscow has not responded to CNN's request for comment on this subject, and there is no way know exactly what is going on hidden from the view of satellites.

Increased activity was also detected at the Chinese nuclear test site in Lop Nur, a dried up salt lake between two deserts in the sparsely populated western China.

Satellite images show a new, fifth underground tunnel has been under excavation in recent years, and fresh roads have been built. A comparison of the images taken in 2022 and 2023 shows the spoil pile has been steadily increasing in size, leading analysts to believe tunnels are being expanded, Lewis said.

In addition, the main administration and support area has seen new construction projects. A new storage area was built in 2021 and 2022, which could be used for storing explosives, he added.

"The Chinese test site is different than the Russian test site," Lewis said. "The Chinese test site is vast, and there are many different parts of it."

"(It) looks really busy, and these things are easily seen in satellite imagery. If we can see them, I think the US government certainly can," he added.

Increased activity at Lop Nur was also noted in an April report by the Sasakawa Peace Foundation's China Observer project, a group of China experts in Japan.

After an analysis of satellite photos of the Lop Nur site, the group concluded that China's "possible goal is to conduct subcritical nuclear tests."

It found a possible sixth testing tunnel under construction at Lop Nur, saying "the fact that a very long tunnel has been dug along the mountain's terrain with bends on the way indicates that the construction of the test site is in its final phase."

In a statement to CNN, China's Foreign Ministry criticized the report as "hyping up 'China's nuclear threat'," and described it as "extremely irresponsible."

"Since the announcement of suspending nuclear tests in 1996, the Chinese side has consistently respected this promise and worked hard in defending the international consensus on prohibiting nuclear testing," it said.

It added that the international world should have "high vigilance" about the United States' activities in nuclear testing.

Activity in Nevada's desert

The US releases an unclassified version of the Nuclear Posture Review every few years, which provides an overview of the role of nuclear weapons in its security strategy.

The most recent report, released in October last year, said that Washington would only consider using nuclear weapons in "extreme circumstances." However, it also stated that the US does not adopt a "no first use policy" because it would result in an "unacceptable level of risk" to its security.

The US conducted its last underground test in 1992, but Lewis said the US has long been keeping itself in a state of readiness for a nuclear test, ready to react if one of its rivals moves first.

"The United States has a policy of being prepared to conduct a nuclear test on relatively short notice, about six months," he said.

The commercial satellite imagery, taken above the nuclear test site in Nevada, officially known as the Nevada National Security Site, shows that an underground facility -- the U1a complex -- was expanded greatly between 2018 and 2023.

The National Security Administration (NNSA), an arm of the US Department of Energy that oversees the site, says the laboratory is for conducting "subcritical" nuclear experiments, a longstanding practice meant to ensure the reliability of weapons in the current stockpile without full-scale testing.

"In subcritical experiments, chemical high explosives generate high pressures, which are applied to nuclear weapon materials, such as plutonium. The configuration and quantities of explosives and nuclear materials are such that no nuclear explosion will occur," the NNSA's website says.

In response to CNN's request for comment, a spokesperson from the NNSA confirmed it has been "recapitalizing infrastructure and scientific capabilities" at the Nevada test site, which includes procuring new advanced sources and detectors, developing reactivity measurement technology, and continuing tunneling activity.

"(This) will provide modern diagnostic capabilities and data to help maintain the safety and performance of the US nuclear stockpile without further underground nuclear explosive testing," the spokesman added.

A report from the US Governmental Accountability Office (GAO) released in August says the US will build two measurement devices at the Nevada site to "make new measurements of plutonium during subcritical experiments."

The devices and related infrastructure improvements, needed "to inform plans for modernizing the nuclear weapons stockpile" will cost about $2.5 billion to $2.6 billion and be ready by 2030, according to the GAO report.

A spokesman from the National Security Council also told CNN that it is closely monitoring Russia's military activities, but added it has "not seen any reason to adjust our own nuclear posture."

However, the expansion of facilities at the Nevada test site could fuel fears in Moscow and Beijing that Washington may be preparing for a nuclear test -- because while both countries could see the development from satellite images, they lack the ability to independently verify what's going on inside, Lewis said.

And such perceptions can become dangerous, especially in the current era with fear and lack of trust on all sides, he said.

"The danger is even if all three start by only planning to go second, one of them might talk themselves into the importance of going first, one of them might decide that since everybody else is doing it, it's better to get the jump and really get going."

If they do, the world would know -- any major underground blast is likely to be detected by the International Monitoring System (IMS), a network of 337 facilities that monitors the planet for signs of nuclear explosions.

Continued modernization

Hans Kristensen, the director of the Nuclear Information Project at the Federation of American Scientists, agreed there is a real danger of testing escalation should one of the major powers do so.

"The minute one of the major nuclear powers pops a nuclear weapon somewhere, you know, all bets are off, because there's no doubt that everyone will join that business again," he said.

In a recent yearbook on world nuclear forces, co-authored by Kristensen and published by the Stockholm International Peace Research Institute (SIPRI) in June, analysts concluded that all of the world's nuclear powers -- which also included the United Kingdom, France, India, Pakistan, North Korea and Israel -- have continued to "modernize their nuclear arsenals" last year.

Russia, for instance, announced on September 1 that its new Sarmat or "Satan II" intercontinental ballistic missile is operational. The Sarmat could carry 10 and possibly more independently targeted nuclear warheads with a range of up to 18,000 kilometers (or about 11,185 miles), according to the Missile Defense Project at the Center for Strategic and International Studies.

The US is also building new delivery systems for nuclear warheads like the B-21 stealth bomber and Columbia-class ballistic missile submarine. As part of the upgrade, nuclear storage sites will also be added to US Air Force bases in Ellsworth and Dyess, Kristensen wrote in a report in the Federation of American Scientists in 2020.

The SIPRI report said that Russia and the US currently possess about 90% of all nuclear weapons in the world, with the US estimated to have more than 3,700 warheads stockpiled, and Russia having about 4,500. Both countries keep their strategic nuclear arsenals on "hair-trigger" alert, meaning that nuclear weapons can be launched on short notice.

China's nuclear arsenal has increased from 350 warheads in January 2022 to 410 in January 2023.

In the past, China did not marry up warheads with delivery systems, keeping their nuclear forces on a "low-alert" status. But the Arms Control Association (ACA) NGO said this year the PLA now rotates missile battalions from stand-by to ready-to-launch status monthly.

Fiona Cunningham, a nonresident scholar in the Nuclear Policy Program at the Carnegie Endowment for International Peace, wrote in the ACA's monthly journal in August that Beijing's nuclear stance is hard to discern.

"The increasing size, accuracy, readiness, and diversity of China's arsenal bolsters the credibility of the country's ability to threaten retaliation for a nuclear strike and enables China to make more credible threats to use nuclear weapons first," she wrote.

But Kristensen told CNN that while all three major powers have been engaging in subcritical tests, he believed "a full-scale nuclear test is unlikely."

Daryl Kimball, executive director of the Arms Control Association, agreed, writing in the organization's September newsletter that "China, Russia, and the United States continue to engage in weapons-related activities at their former nuclear testing sites."

But Kimball noted that without a real test, "it is more difficult, although not impossible, for states to develop, prove, and field new warhead designs."

What's the point of more tests?

But if all three countries have suspended nuclear testing since the 1990s, what could they gain from the resumption of these tests?

Lewis said a reason to test, especially for China, is to get more up-to-date data for computer models that show what a nuclear explosion will do. Because while the United States and Russia have conducted hundreds of tests, China has only done around 40 and has significantly fewer data points.

"Those 40 tests were done in the 1960s, in the 1970s, in the 1980s, when their technology wasn't that high. The data that you have is not that good," Lewis said.

Others point out that the big powers have not tested low-yield nuclear weapons, which produce a smaller nuclear explosion that might be targeted on a specific battlefield unit or formation, rather than destroying a major city.

In a 2022 report for the Johns Hopkins Applied Physics Laboratory in Baltimore, researchers Michael Frankel, James Scouras and George Ullrich suggest that the US might hesitate to retaliate for a Russian low-yield attack because it has not tested the types of weapons it would need to use.

"While the United States now has several lower-yield weapons in its arsenal, they are insufficient in quantity and diversity of delivery systems," their report, titled "Tickling the Sleeping Dragon's Tail," says.

In particular, the report says, smaller nukes, with yields lower than a kiloton (for comparison, the atomic bomb the US dropped on Hiroshima, Japan, in 1945 had a yield of about 15 kilotons) that can be delivered by aircraft or ships have been proposed a deterrent to Russian nuclear threats.

"Such weapons are unlikely to be available absent testing," the report says.

The United States, the world's first nuclear power, has conducted 1,032 tests, the first coming in 1945 and the last coming in 1992, according to the United Nations' data. The Soviet Union -- now Russia -- conducted 715 between 1949 and 1990, and China has tested 45 times between 1964 and 1996.

Lewis believed an urge for the US, Russia and China to be the first to develop "exotic" weapons of the future also instills a need for nuclear testing of those possible systemsl.

Some of these may soon be in the Russian arsenal, as Putin has boasted about weapons like an nuclear-armed doomsday torpedo and a nuclear-powered cruise missile.

"We're on the verge of this kind of science fiction future where we are resurrecting all of these terrible ideas from the Cold War," Lewis said.