US Navy Adm. John Richardson shakes hands with Japan’s Prime Minister Shinzo Abe in Tokyo, Thursday. US naval vessels sailing through the South China Sea, including areas claimed by China, cannot be considered provocative, the US Navy’s most senior uniformed officer said.

Beijing warned Thursday that it would “firmly oppose” infringement of its sovereignty after indications Washington will soon send warships close to its artificial islands in the South China Sea.

Tensions have mounted since China transformed reefs in the area — also claimed by several neighboring countries — into small islands capable of supporting military facilities, a move that the US says threatens freedom of navigation.

Senior officials in Washington have signaled that the US military could sail close by the islands in the coming days or weeks to demonstrate that Washington does not recognize a Chinese claim to territorial waters around them.

Chinese foreign ministry spokeswoman Hua Chunying said that the country respected freedom of navigation and overflight in the South China Sea, but would “firmly oppose infringement of sovereignty under that pretext.”

Beijing insists it has sovereign rights to nearly all of the South China Sea, even waters close to the coasts of other states.

The sea is a strategically vital waterway with shipping lanes through which about a third of all the world’s traded oil passes, and the dispute has raised fears of clashes.

Hua’s comments came after US Defense Secretary Ash Carter warned Beijing following a meeting of American and Australian officials Tuesday that Washington will continue to send its military “wherever international law allows,” including the South China Sea.

Australia is a key Pacific ally of the US and its foreign minister Julie Bishop said the two countries were “on the same page” on the issue.

An editorial in the Global Times, which is close to China’s ruling Communist party, condemned Washington’s “ceaseless provocations and coercion.”

“China mustn’t tolerate rampant US violations of China’s adjacent waters and the skies over those expanding islands,” it said, adding that its military should “be ready to launch countermeasures according to Washington’s level of provocation.”

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