The warning from the U.S. and its four closest intellignce partners follows several arrests of U.S. service members for allegedly trying to pass info to China.

The U.S. and its intelligence partners warned on Wednesday that China is working to recruit current and former Western military pilots and other service members to help strengthen Chinese air power and gain insights into Western aviation tactics.

Describing China’s efforts as a “persistent” threat, the warning came in a joint bulletin from the so-called Five Eyes intelligence partners, five nations that share sensitive intelligence: the U.S., the United Kingdom, Canada, Australia and New Zealand.

China’s People’s Liberation Army (PLA) “is using private companies in South Africa and China to hire former fighter pilots from Canada, France, Germany, the United Kingdom, Australia, the United States, and other Western nations to train PLA Air Force and Navy aviators,” the bulletin said.

“The PLA wants the skills and expertise of these individuals to make its own military air operations more capable while gaining insight into Western air tactics, techniques, and procedures. The insight the PLA gains from Western military talent threatens the safety of the targeted recruits, their fellow service members, and U.S. and allied security.

“This threat continues to evolve in response to Western government warnings to their military personnel and public, so this notice seeks to continue highlighting this persistent, adaptive threat,” it said.

The bulletin said the U.S. and its Western partners have taken steps to try to counter the threat, including imposing commercial restrictions on a test flying academy in South Africa and Chinese organizations linked to the spying operations, and prohibiting former military members from working with China.

The bulletin advised current and former Western military members to be on the alert for possible approaches, including job offers from privately owned companies that hide their links to China.

NATO members in January held a conference of senior officers and officials to examine how to counter China’s spying efforts.

The warning from the intelligence partners comes amid growing concern about China’s military buildup and recent drills around Taiwan, which Beijing described as “punishment” after the self-governing island held elections. China says Taiwan is part of its territory and has vowed to take control of the island, by force if necessary.

Wednesday’s bulletin also follows a series of cases in which U.S. military service members have been charged with trying to sell sensitive information to China.

In 2022, former Marine Corps fighter pilot Daniel Duggan was arrested in Australia for allegedly training PLA pilots how to land on aircraft carriers. Duggan, who denies the charges, is awaiting extradition to the U.S.

In March, U.S. Army intelligence analyst Korbein Schultz was arrested and charged with conspiring to sell sensitive defense information to China. He has pleaded not guilty. And in August last year, two Navy sailors were arrested for allegedly relaying sensitive military information to China. Wenheng Zhao pleaded guilty and was sentenced to 27 months. Jinchao Wei has pleaded not guilty.

“These arrests are a reminder of the relentless, aggressive efforts of the People’s Republic of China to undermine our democracy and threaten those who defend it,” assistant director of the FBI’s Counterintelligence Division Suzanne Turner said at the time.

A spokesperson for the Chinese Embassy in Washington expressed unfamiliarity with the specific allegations and declined to comment, but said in a general statement, “In recent years, some U.S. officials have been quick to accuse China and smear the normal exchanges and cooperation between China and the U.S., which is not conducive to the healthy development of China-U.S. relations.”