China said on Tuesday that it has expressed concern to the U.S. over what it considered an affront to its sovereignty after two U.S. warships sailed through the Taiwan Strait.

The move adds to increasingly fraught relations between the two countries, which have clashed over a number of issues, including trade, Beijing’s territorial claims in the South China Sea and human rights abuses in Xinjiang.

Monday was the second time in the space of three months that American warships had conducted so-called “freedom of navigation” exercises in the Taiwan Strait, a 180-km wide stretch of water separating the Chinese mainland and the self-ruled island.

Beijing “expressed its concern to the U.S. side” as “the Taiwan issue concerns China’s sovereignty and territorial integrity”, Chinese Foreign Ministry spokeswoman Hua Chunying told a regular news briefing.

Colonel Rob Manning from the U.S. Department of Defense told reporters that the USS Curtis Wilbur and USS Antietam conducted a routine transit to demonstrate U.S. commitment “to a free and open Indo-Pacific”. Multiple Chinese warships shadowed the two U.S. vessels during the transit, following at a safe distance, American defence officials told CNN. Washington remains Taiwan’s most powerful unofficial ally and its main arms supplier despite switching diplomatic recognition to Beijing in 1979.

The Taiwan issue concerns China’s sovereignty and territorial integrity

Hua Chunying

Spokeswoman, Foreign Ministry

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