The U.S. challenged once again China's claim to the South China Sea on Thursday by flying two B-1B Lancer bombers over the contested waters, the Air Force said.

The move risks irking China, who has made claims to the sea and has built up small islands and military installations in the area.

President Trump is set to meet with Chinese President Xi Jinping on the sidelines of the Group of 20 summit in Hamburg, Germany.

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Potentially the most high-profile issue the two leaders will discuss when they meet is North Korea's recent test of an intercontinental ballistic missile — the reclusive government's first successful launch of such a weapon.

Trump has pressed China to do more to rein in North Korea. But he has acknowledged himself in recent weeks that Beijing has done little to curb Pyongyang's rapidly advancing nuclear and missile programs.

The flyover of the South China Sea on Thursday was the latest move by the U.S. to challenge China's claim in the region.

Chinese Foreign Ministry spokesman Geng Shuang told Reuters that there was nothing preventing the U.S. from navigating the area by either sea or air. But he also ripped "individual countries using the banner of freedom of navigation and overflight to flaunt military force and harm China's sovereignty and security."

China's Defense Ministry said in a statement to Reuters that the country was monitoring "relevant countries' military activities next to China."