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.
On Saturday, a statement from China’s defence ministry accused the US of deliberately raising tensions in the area with the B-52 over-flight of the disputed Spratly Islands, which it calls Nansha.
A report in the Wall St Journal said there were two B-52s on the mission and that one unintentionally flew within two nautical miles of Cuarteron Reef on the Spratly Islands, possibly due to bad weather.
China said the flights “constitute a serious military provocation and are rendering more complex and even militarising conditions in the South China Sea”.
It urged the US to take measures to prevent similar incidents.
The US has not taken sides on sovereignty issues in the area but has a “freedom of navigation” policy asserting right of passage for its military.
However, Pentagon spokesman Cmdr Bill Urban said the B-52 over-flight was not part of this policy, which analysts say might suggest a navigation error.