The manoeuvre saw around 10 American F-16 fighter jets fly into an area over the Yellow Sea between the South Korean and Chinese air defence identification zones, prompting Beijing to dispatch its own fighters to the site, according to South Korean media reports.

No clashes occurred, but Defence Minister Ahn Gyu-back protested that South Korea had not been properly informed about the exercise, the reports said.

US Forces Korea (USFK) said Wednesday its commander had spoken to Ahn and "expressed regret" that Seoul's defence officials had not been briefed on the drill "in time".

USFK "conducts regular training to maintain the highest level of readiness and ensure it can fulfill its mission", it said in a statement.

It did not acknowledge whether the standoff with Chinese jets took place.

The United States is a longstanding security ally of South Korea and currently stations 28,500 troops in the country as a deterrent against its nuclear-armed adversary North Korea.

China is a crucial ally of North Korea. It has also frequently butted heads with Washington over support for Taiwan, the self-ruled island it claims as its own.

US President Donald Trump said last week he would decide soon on whether to send more weapons to Taiwan, after Chinese President Xi Jinping warned him not to do so.

South Korean President Lee Jae Myung has notably stayed out of a growing spat between China and Japan over Prime Minister Sanae Takaichi's suggestion that Tokyo could intervene militarily if Beijing attacked Taiwan.