China said yesterday it has expressed concern to the United States 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-kilometre wide stretch of water separating the Chinese mainland and Taiwan.

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. Hua said Beijing urged Washington to "scrupulously abide by the one-China principle" and "carefully handle the Taiwan-related issues in an appropriate manner."

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 defense officials told CNN.

Beijing recently conducted a series of military maneuvers, including a live fire exercise in the Taiwan Strait in April, declaring its willingness to confront Taiwan's "independence forces."

Washington remains Taipei's most powerful unofficial ally and its main arms supplier despite switching diplomatic recognition to Beijing in 1979. The Trump administration has sought closer ties to the island, announcing plans last month to sell it $330 million spare parts for several aircraft including the F-16 fighter and the C-130 cargo plane. The arms sale coincided with a U.S. decision to issue a visa ban and assets freeze on China's Equipment Development Department and its director, Li Shangfu, over the purchase from Russia of Su-35 combat aircraft in 2017 and S-400 surface-to-air missile system-related equipment this year.

China's purchase of the weapons from Rosoboronexport, Russia's main arms exporter, violated a 2017 law intended to punish the government of Russian President Vladimir Putin for interfering in U.S. elections and other activities. In response, China demanded the sanctions be revoked, summoned the American ambassador and defense attaché to deliver a protest, and recalled its naval commander from a U.S. trip.

China also turned down a request for an October port call in Hong Kong by the U.S. Navy amphibious assault ship USS Wasp. China last denied such a visit in 2016 amid a spike in tensions between the countries over the South China Sea.