SEOUL, South Korea (AP) — North Korean leader Kim Jong Un was to travel to the far eastern port city of Vladivostok to see Russia’s Pacific fleet, while his official media back home said Saturday that he was “deeply impressed” by a factory producing the most advanced Russian warplanes.

Kim’s trip to Russia, highlighted by a summit with Russian President Vladimir Putin on Wednesday, has sparked Western concerns about an arms alliance in which North Korean munitions fuel Putin’s war in Ukraine and Russian technologies advance the threat posed by Kim’s military nuclear program.

After meeting with Putin at Russia’s main spaceport, a location that communicated Kim's desire for Russian assistance in his efforts to acquire space-based reconnaissance assets and missile technologies, North Korea's leader reappeared Friday in the far eastern city of Komsomolsk-on-Amur for a visit to a plant producing Russia’s Su-57 fighter jets.

Experts have said potential military cooperation between the countries could include efforts to modernize North Korea’s outdated air force, which relies on warplanes sent from the Soviet Union in the 1980s.

Kim’s plans to see Russian naval ships in Vladivostok could be another hint at what he wants from Russia, possibly in exchange for supplying munitions to refill Putin’s declining reserves as his invasion of Ukraine becomes a drawn-out war of attrition.

Kim in recent months has been emphasizing the need to strengthen his navy to counter the advanced naval assets of the United States, which has been expanding its combined military exercises with South Korea to counter the North’s growing threat.

Analysts said Kim’s focus on naval strength could be driven by ambitions to obtain sophisticated technologies for ballistic missile submarines and nuclear-propelled submarines as well as to initiate joint naval exercises by Russia and North Korea.

Pyongyang’s official Korean Central News Agency said Saturday that during his visit to the aircraft plant in Komsomolsk-on-Amur, Kim expressed “sincere regard” for what he described as Russia’s rapidly advancing aviation technologies.

North Korean state media have been reporting Kim’s activities in Russia a day late while crafting the details to meet government propaganda purposes.

Russia’s Cabinet on Friday released a video showing Kim on an elevated platform looking at the cockpit of an Su-57 while listening to its pilot. Kim also beamed and clapped his hands when an Su-35 fighter jet landed after a demonstration flight.

During a luncheon hosted by Russian officials, Kim’s top military officer, army Marshal Ri Pyong Chol, said his leader’s visit to the facility “added another glorious page” to the relations between the countries, KCNA said. Kim's delegation also includes the top commanders of North Korea's air force and navy.

Putin on Friday briefed Belarusian President Alexander Lukashenko about his summit with Kim. During their meeting in the Black Sea resort of Sochi, Lukashenko suggested Belarus could join Russia and North Korea in “three-way cooperation.”

Kim’s trip to Russia, his first since April 2019 when he met Putin in Vladivostok, came days after he attended a ceremony at a North Korean military shipyard where the country unveiled a purported nuclear attack submarine.

State media claimed it is capable of launching tactical nuclear weapons from underwater. But South Korea’s military expressed doubt about the operational capabilities of the sub, which was the result of reshaping an existing submarine to install missile launch tubes.

Kim has announced goals to acquire nuclear-propelled submarines, which can quietly travel long distances and approach enemy shores to deliver strikes, a key asset in his efforts to build a viable nuclear arsenal that could threaten the United States. Analysts say such capacities would be unfeasible for the North without external assistance.

Putin on Friday reiterated that Russia would abide by U.N. sanctions, some of which ban North Korea from exporting or importing any weapons. Kremlin spokesperson Dmitry Peskov separately said that no agreements on bilateral military cooperation were signed after the Putin-Kim meeting Wednesday.

Experts say North Korea and Russia aren’t likely to publicize any deals on weapons to avoid stronger international criticism.

Since last year, the U.S. has accused North Korea of providing ammunition, artillery shells and rockets to Russia, likely much of them copies of Soviet-era munitions. South Korean officials said North Korean weapons provided to Russia have already been used in Ukraine.

Some analysts question how much Russia would be willing to share its sensitive weapons technology in exchange for North Korean conventional arms. But others say that is now a possibility to consider as Russia becomes desperate to refill its drained reserves.

After a meeting in Seoul discussing the allies’ nuclear deterrence strategies, U.S. and South Korean officials on Friday stepped up their condemnation of the recent moves by Russia and North Korea.

Sasha Baker, the U.S. acting undersecretary of defense for policy, said Washington will continue to “try to identify and expose and counter Russian attempts to acquire military equipment, again, to prosecute their illegal war on Ukraine.”

South Korean Vice Foreign Minister Chang Ho-jin said Washington and Seoul, while increasing security cooperation, would ensure that Moscow faces consequences if it helps advance North Korea’s weapons program.