The US is being accused of flying a B52 bomber close to an artificial island in the disputed South China Sea in an act of “provocation.”
It is alleged to have taken place near Cuarteron Reef in the Spratly Islands – disputed territory claimed by China and several of its neighbours.
Beijing claims the plane flew within two nautical miles and it was warned to leave.
The Pentagon says its looking into the complaint, but that there was no plan for the B52 to fly within 12 nautical miles of any artificial island.
One US defence official, speaking on condition of anonymity, said it was believed the plane was further away than two nautical miles, but had mistakenly come within 12 nautical miles.
In October, a US guided missile destroyer sailed close to Subi Reef. It was a deliberate move to challenge China’s claims of territorial waters.
Tensions regularly flare over the South China Sea, where several countries have overlapping claims.
China claims almost all of the energy-rich waters, through which trillions of euros of maritime trade passes each year. The Philippines, Brunei, Vietnam, Malaysia and Taiwan also have claims.
The US has warned of a possible arms race, as nations become more tempted to use military force to settle their spats.