The United States says China is its most consequential challenge, and maintaining a military edge aids Washington's efforts to deter aggression by Beijing.

"The People's Liberation Army (PLA) wants to exploit your knowledge and skill to fill gaps in their military capability," Brown -- who has been nominated to become the top US military officer -- wrote in a document addressed to members of the Air Force.

"Foreign companies are targeting and recruiting US and NATO-trained military talent across specialties and career fields to train the PLA abroad," Brown said.

"By essentially training the trainer, many of those who accept contracts with these foreign companies are eroding our national security, putting the very safety of their fellow service members and the country at risk, and may be violating the law."

China and the United States in recent months have resumed dialogue with hopes of bringing greater stability to the turbulent relationship between the two countries.

Tensions soared this year over issues including an alleged Chinese spy balloon that was shot down by a US warplane after traversing the country, and competing interests in the democratic self-ruled island of Taiwan, which Washington supports militarily while Beijing claims it as part of its territory.