China has accused the US of "serious provocation" after it flew B-52 bombers near one of the disputed Spratly Islands in the South China Sea.
Chinese military personnel were put on "high alert" during the incident on 10 December, and issued warnings to leave.
The Pentagon said it was looking into the complaint.
China claims large swathes of the South China Sea but is in territorial dispute with a host of regional neighbours.
In October, China rebuked the US after a destroyer sailed close to a reef.
photo credits: Reuters