Two US B-52 bombers flew close to islands in the flashpoint South China Sea last week in a "serious military provocation", Beijing said Saturday, as tensions simmer in the disputed waterway.

China insists it has sovereignty over virtually all of the resource-rich sea, conflicting with the various claims of several neighbouring nations, and US activity in the area has provoked Beijing's ire several times in recent months.

"In the morning of 10 December, two US B-52 bombers entered air space over the Chinese Nansha islands and nearby areas without authorisation," Beijing's defence ministry said, using the Chinese name for the Spratly Islands.

"This behaviour is a serious military provocation which complicates the general situation in the South China Sea, (contributing) to the militarisation of the region," the ministry said in a statement on its website.

During a mission by the two B-52 bombers last week, one of the aircraft unintentionally flew within two nautical miles of an artificially constructed island, the Wall Street Journal quoted Pentagon officials as saying Friday.

This may have been because of bad weather conditions, according to officials quoted in the newspaper.

Beijing's defence ministry added: "The United States has continuously sent military ships and planes to make a show of force and create tensions in the waters and airspace" of the South China Sea.

"The Chinese army will take all necessary measures to defend the sovereignty and the security of the country."

The United States is critical of China building artificial islands in the disputed sea, and has flown other B-52 bombers and sailed a guided-missile destroyer near some of the constructions in recent months.

Washington has said China's transformation of the geographical features in the Spratly Islands poses a threat to freedom of navigation in the critical body of water.

China's military conducted war games in the area this week, with warships, submarines and fighter jets deployed over a "range of several thousand kilometres", the People's Liberation Army Daily said.

Beijing insists it has sovereignty over virtually all of the resource-rich South China Sea, conflicting with the various claims of Vietnam, Malaysia, the Philippines, Taiwan and Brunei.