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 US and its closest intelligence partners are warning that China has intensified a campaign to recruit former Western military pilots and other service members to train its own aviators, according to a joint bulletin released by the nations on Wednesday.

Beijing has for years struggled with its air operations — including both pilot proficiency and its technical capabilities to counter western aviation tactics — even as it has built up its air and naval forces in preparation for a possible military takeover of the island of Taiwan.

Western-trained pilots can teach Chinese military personnel everything from air combat tactics to how to land on an aircraft carrier, according to a US official. They can also give China a precious window into how it can counter western combat tactics.

The warning was issued by the so-called Five Eyes nations, which share higher-level intelligence among them: the US, the UK, Canada, Australia and New Zealand.