U.S. warplanes have flown over the disputed South China Sea, the U.S. Air Force said Friday, July 7, a move aimed at asserting freedom of navigation rights in the hotly-contested area, AFP reports.

The flights come as both U.S. President Donald Trump and Chinese President Xi Jinping attend the G-20 meeting in Hamburg.

A U.S. B1-B bomber and two F-15 fighters with the Japan Air Self-Defense Force departed Guam's Andersen Air Force Base and flew over the neighbouring East China Sea late on Thursday, the U.S. Pacific Air Forces Public Affairs office said.

This is "the first time U.S. Pacific Command-directed B-1B Lancers have conducted combined training with JASDF fighters at night," the statement read.

When the bilateral operations concluded, "the B-1Bs proceeded to the South China Sea before returning to Andersen Air Force Base," the statement read.

It was not immediately clear when or if a second U.S. B1-B bomber joined for the South China Sea leg of the operation.

The mission "demonstrates how the U.S. will continue to exercise the rights of freedom of navigation anywhere international law allows," the U.S. statement read.

"Flying and training at night with our allies in a safe, effective manner is an important capability shared between the U.S. and Japan," said Major Ryan Simpson, Pacific Air Forces chief of bomber operations.

The joint military flight demonstrates U.S.-Japanese "solidarity ... to defend against provocative and destabilizing actions in the Pacific theater," the statement added.

Beijing claims nearly all of the South China Sea, parts of which are also claimed by Taiwan and Southeast Asian nations including the Philippines, Brunei, Malaysia and Vietnam.

China has rapidly built reefs in the area into artificial islands capable of hosting military planes.

China also lays claim to tiny islands in the East China Sea between Japan and the Korean peninsula, and between Japan and Taiwan.