The US committed "a serious military provocation" by flying two Air Force B-52 bombers over a Chinese-controlled man-made island in the South China Sea, China's Defence Ministry says.

The Americans had suggested the planes had strayed off course.

Chinese officials has accused the US of deliberately raising tensions in the disputed region, where China has been aggressively asserting its claims to virtually all islands, reefs and their surrounding seas. It also reiterated that it would do whatever is necessary to protect China's sovereignty.

The Foreign Ministry took a more diplomatic tone, saying the situation was stable.

The US takes no official stance on sovereignty claims in the strategically crucial South China Sea, through which five trillion dollars trillion in international trade passes each year. However, Washington insists on freedom of navigation and maintains that China's seven newly created islands do not enjoy traditional rights, including a 12-nautical-mile (22-km) territorial limit.

Responding to China's protests, Pentagon spokesman Mark Wright said that the December 10 mission was not a "freedom of navigation" operation and that there was "no intention of flying within 12 nautical miles of any feature," indicating the mission may have strayed off course.

The US uses pre-planned freedom of navigation operations to assert its rights to "innocent passage" in other countries' territorial waters.

"The United States routinely conducts B-52 training missions throughout the region, including over the South China Sea," Mr Wright said. "These missions are designed to maintain readiness and demonstrate our commitment to fly, sail and operate anywhere allowed under international law."

He said the US was "looking into the matter".

China's Defence Ministry demanded that Washington immediately take measures to prevent such incidents and damage to relations between the two nations' armed forces.

"The actions by the US side constitute a serious military provocation and are rendering more complex and even militarising conditions in the South China Sea," the ministry said.

It said Chinese military personnel on the island went on high alert during the overflights by the B-52s and that they issued warnings demanding the aircraft leave the area.

China's latest protest comes amid a simmering dispute over Washington's approval this week of the first arms package in four years offered to Taiwan, Beijing's self-governing rival.