HONG KONG (AP) — Hong Kong’s leader on Tuesday said the sentencing on spying charges of a U.S. citizen in China, who was also a permanent resident of the semi-autonomous city, illustrated that the territory should “stay vigilant to national security risks hidden in society.”

The government said mainland Chinese authorities had informed Hong Kong of the arrest of John Shing-Wan Leung in 2021. He was sentenced Monday to life in prison. Hong Kong's government was prepared to provide assistance to anyone arrested by mainland authorities upon request but had not yet received any in Leung's case, authorities said.

Neither Hong Kong Chief Executive John Lee nor the court in the eastern Chinese city of Suzhou that tried Leung, 78, have released details of his alleged crime.

Since taking office last year, Lee has taken a hard line toward any signs of dissent, backed up by the uncompromising attitude adopted by Chinese leaders from authoritarian Communist Party head Xi Jinping on down.

“This incident showed us that national security risks could be hidden in society. That’s why we repeatedly stressed that, although Hong Kong’s situation appears to be largely stabilized, we can’t let down our guard over national security risks,” said Lee, a former police officer and head of security in the city.

A longtime Beijing loyalist, he was effectively appointed to the top position after running unopposed in an election choreographed by Beijing last year.

In an echo of party propaganda, Lee referred to the 2019 pro-democracy protests that triggered a crackdown as “black violence” and Hong Kong’s version of “color revolution,” a phrase used by China and Russia to describe political movements seeking to overturn authoritarian regimes. Lee said the protest movement was an alarm bell that reminds the city to keep monitoring such risks.

Leung's sentencing threatens to further exacerbate already strained ties between Beijing and Washington.

Leung was detained April 15, 2021, by the local bureau of China’s counterintelligence agency in Suzhou, according to a statement posted by the city’s intermediate court on its social media site. His detention came after China had closed its borders and imposed tight domestic travel restrictions and lockdowns affecting tens of millions to fight the spread of COVID-19.

Such investigations and trials are held behind closed doors and little information is generally released.

The harsh sentence given Leung was especially notable because of his previous affiliations with pro-Communist Party organizations, including one seeking overseas support for Beijing's goal of unification with self-governing Taiwan.

Relations between Washington and Beijing are at their lowest in decades amid disputes over trade, technology, human rights and China’s increasingly aggressive territorial claims toward Taiwan, the South China Sea and elsewhere.

High-level government exchanges between the sides have been placed on hold and U.S. companies are delaying major investments amid mixed messaging from Beijing. Many Chinese firms, most notably telecoms giant Huawei, have been effectively shut out of the U.S. market due to legal bans and high tariffs.

The sentencing comes as U.S. President Joe Biden is traveling to Hiroshima, Japan this weekend for the summit of the Group of Seven major industrial nations, followed by a visit to Papua New Guinea, a Pacific island nation in a region where China has sought to expand its economic, military and diplomatic influence.

While the Suzhou court offered no indication of a link between Leung's case and overall China-U.S. relations, spying charges in China often appear highly selective and evidence backing them up is held in secret. The party's rigid control over courts, civil society and the media effectively blocks efforts to gain further information or mount legal appeals.

The U.S. Embassy in Beijing said Monday it was aware of Leung's case, but could not comment further due to privacy concerns. “The Department of State has no greater priority than the safety and security of U.S. citizens overseas,” the embassy said.

A former British colony, Hong Kong was promised it would retain its financial, social and political liberties when returned to China in 1997. Beijing has since torn up that commitment through progressively harsher restrictions on public gatherings, free speech and political participation, while still promoting the city as an efficient and corruption-free center for trade and finance.

Meanwhile, on the mainland, Chinese national security agencies have raided the offices of foreign business consulting firms in Beijing and other cities as part of a crackdown on foreign businesses that provide sensitive economic data.

The pressure on foreign companies appears to clash with attempts by Beijing to lure back foreign investors after draconian COVID-19 pandemic restrictions were lifted at the beginning of the year.

It wasn't clear who represented Leung at his trial and his family has not commented on the sentence. Friends and former colleagues declined The Associated Press' requests for comment.

Long pretrial detentions are not unusual in China and prosecutors have broad powers to hold people charged in national security cases, regardless of their citizenship status.

Two Chinese-Australians, Cheng Lei, who formerly worked for China’s state broadcaster, and writer Yang Jun, have been held since 2020 and 2019 respectively, without word on their sentencing.

Government suspicion is particularly focused on Chinese-born foreign citizens and people from Taiwan and Hong Kong, especially if they have political contacts or work in academia or publishing.

Under Xi, the party has launched multiple campaigns against what it calls foreign efforts to sabotage its rule, without showing evidence. Online commentary and independent information sources have been muzzled and universities ordered to censor discussions of human rights, modern Chinese history and ideas that could prompt questions about total Communist Party control.

Xi’s government has also taken a hard line on foreign relations, most recently ordering a Canadian diplomat to leave at short notice in retaliation for Ottawa’s expulsion of a staffer at the Chinese Embassy who was accused of threatening a member of the Canadian Parliament and his family members living in Hong Kong.

China's leader for a decade who faces no term limits, Xi has taken a highly confrontational stance toward the U.S. and other democracies, while backing Russian President Vladimir Putin in his invasion of Ukraine and supporting other autocratic governments from Nicaragua to Myanmar.