Two U.S bombers flew over the disputed South China Sea, the U.S. Air Force said on Friday, asserting the right to treat the region as international territory despite China's claims in the busy waterway.

The flight from Guam on Thursday came as U.S. President Donald Trump and Chinese President Xi Jinping prepare for a likely meeting on the sidelines of the G20 summit in Germany. The two leaders were expected to discuss what Beijing can do to rein in Pyongyang's missiles and nuclear weapons programmes.

The United States believes North Korea fired an intercontinental ballistic missile on Tuesday that put parts of the U.S. mainland within range of Pyongyang's warheads for the first time.

China claims nearly all of the South China Sea, through which about $5 trillion in ship-borne trade passes each year, a stance contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam. Beijing usually protests against freedom of navigation operations such as bomber flights.

The United States has criticised China's build-up of military facilities in the South China Sea, concerned that they could be used to extend Beijing's strategic reach.

The two Lancers that made the latest flight had earlier trained with Japanese jet fighters in the neighbouring East China Sea, the first time the two forces had conducted joint night-time drills.

"This is a clear demonstration of our ability to conduct seamless operations with all our allies," U.S. Air Force spokesman Major Ryan Simpson said in a statement.

Two U.S. B-1B Lancer bombers flew from Guam over the South China Sea last month, while a U.S. warship carried out a manoeuvring drill within 12 nautical miles of one of China's artificial islands in the waterway in late May.