Two U.S. bombers flew over the South China Sea on Friday as President Trump and China's President Xi Jinping prepare for a possible meeting on the sidelines of the G-20 summit in Germany.
The two US Lancers had earlier trained with Japanese jet fighters in the East China Sea, the first time the two forces had conducted joint night-time drills, said the US Pacific Air Forces public affairs office in a statement.
Two U.S. bombers flew over the highly-contested South China Sea, putting China's extensive claims to the region to the test.
U.S. Pacific Air Forces said the joint drills "demonstrate the solidarity between Japan and the United States to defend against provocative and destabilizing actions in the Pacific theater".
Flying over the East China Sea challenges China's Air Defense Identification Zone (ADIZ), a 200-mile airspace off the coast of China that requires aircraft to identify themselves.
Construction is shown on Mischief Reef, in the Spratly Islands, the disputed South China Sea in this June 19, 2017 satellite image released by CSIS Asia Maritime Transparency Initiative at the Center for Strategic and International Studies (CSIS) to Reuters on June 29, 2017. "But China resolutely opposes individual countries using the banner of freedom of navigation and overflight to flaunt military force and harm China's sovereignty and security", he said.
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China called the action "a serious political and military provocation".
China denies it is militarizing the islands it has built.
Beijing has long laid claim over the waterway, disputing other countries like Taiwan, the Philippines, Brunei, Malaysia and Vietnam, and building up military infrastructure to solidify its presence.
Last week, a $1.4 billion arms sale to Taiwan, which Beijing regards as a renegade province, was approved after months of delay, the same day as China's Dandong bank was sanctioned by the U.S. for alleged ties to North Korea.
The operation comes two months after North Korea launched a ballistic missile toward Japan.