BEIJING —China said on Tuesday it has expressed concern to the United States over what it considered an affront to its sovereignty after two US 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-kilometer wide stretch of water separating the Chinese mainland and the self-ruled democratic island.

Beijing “expressed its concern to the US 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”.

“The ships’ transit through the Taiwan Strait demonstrates the US commitment to a free and open Indo-Pacific,” Commander Nate Christensen, deputy spokesman for US Pacific Fleet, said in a statement.

“The US Navy will continue to fly, sail and operate anywhere international law allows,” he added.

Washington has no formal ties with Taiwan, but is bound by law to help it defend itself and is the island’s main source of arms. The Pentagon says Washington has sold Taiwan more than $15 billion in weaponry since 2010.

China has been ramping up pressure to assert its sovereignty over the island. It raised concerns over US policy toward Taiwan in talks last week with US Defense Secretary Jim Mattis in Singapore.

As the United States prepared for a fresh passage through the strait, it told China’s military that its overall policy toward Taiwan was unchanged.

Mattis delivered that message to China’s Defense Minister Wei Fenghe personally on Thursday, on the sidelines of an Asian security forum.

“Minister Wei raised Taiwan and concerns about our policy. The Secretary reassured Minister Wei that we haven’t changed our Taiwan policy, our one China policy,” Randall Schriver, a US assistant secretary of defense who helps guide Pentagon policy in Asia, told reporters traveling with Mattis.

“So it was, I think, a familiar exchange.”

Taiwan is only one of a growing number of flashpoints in the US-China relationship, which also include a bitter trade war, US sanctions, and China’s increasingly muscular military posture in the South China Sea.

Taiwan’s relations with China have deteriorated since the island’s President Tsai Ing-wen from the independence-leaning Democratic Progressive Party swept to power in 2016.

Beijing, which has never renounced the use of force to bring Taiwan under its control, responded to the July passage with a warning to the United States to avoid jeopardizing “peace and stability” in the strategic waterway.

It has also viewed US overtures toward Taiwan with alarm, including its unveiling a new de facto embassy in Taiwan and passage of the Taiwan Travel Act, which encourages US officials to visit the island. — Agencies