The United States often sends ships and aircraft into the South China Sea to “flex its muscles” and this is not good for peace, China’s Foreign Ministry said on Monday, after a U.S. aircraft carrier group sailed into the disputed waterway.

The strategic South China Sea, through which trillions of dollars in trade flows each year, has long been a focus of contention between Beijing and Washington, with China particularly angered by U.S. military activity there.

The U.S. carrier group led by the USS Theodore Roosevelt and accompanied by three warships, entered the waterway on Saturday to promote “freedom of the seas”, the U.S. military said, just days after Joe Biden became U.S. president..

“The United States frequently sends aircraft and vessels into the South China Sea to flex its muscles,” the foreign ministry spokesman, Zhao Lijian, told reporters, responding to the U.S. mission.

“This is not conducive to peace and stability in the region.”

China has repeatedly complained about U.S. Navy ships getting close to islands it occupies in the South China Sea, where Vietnam, Malaysia, the Philippines, Brunei and Taiwan all have competing claims.

The carrier group entered the South China Sea at the same time as Chinese-claimed Taiwan reported incursions by Chinese air force jets into the southwestern part of its air defence identification zone, prompting concern from Washington.

China has not commented on what its air force was doing, and Zhao referred questions to the defence ministry.

He reiterated China’s position that Taiwan is an inalienable part of China and that the United States should abide by the “one China” principle.

Taiwan President Tsai Ing-wen visited a radar base in the north of the island on Monday, and praised its ability to track Chinese forces, her office said.

“From last year until now, our radar station has detected nearly 2,000 communist aircraft and more than 400 communist ships, allowing us to quickly monitor and drive them away, and fully guard the sea and airspace,” she told officers.

Taiwan’s defence ministry added that just a single Chinese aircraft flew into its defence zone on Monday, an anti-submarine Y-8 aircraft.

Biden’s new administration says the U.S. commitment to Taiwan is “rock-solid”.

The United States, like most countries, has no formal diplomatic ties with Taiwan but is the democratic island’s most important international backer and main arms supplier, to China’s anger.

Reporting by Cate Cadell; Writing and additional reporting by Ben Blanchard in Taipei; Editing by Clarence Fernandez / REUTERS