After a U.S. military plane crossed the strategically important waterway on Monday, China accused the United States of “endangering” peace and stability in the Taiwan Strait. The U.S. Navy responded that the plane had been in international airspace.

Beijing has expressed outrage at U.S. military operations across the strait, which are often conducted by warships but occasionally by aircraft as well, claiming that China “has sovereignty, sovereign rights, and authority” over the waterway. That is disputed by Taiwan and the US, who assert that it is an international waterway.

The Eastern Theatre Command of China’s People’s Liberation Army said its forces closely monitored the P-8A Poseidon maritime patrol and reconnaissance plane, which is also used for anti-submarine missions, as it flew through the strait which separates China from Taiwan.

“The U.S. side’s actions deliberately interfered with and disrupted the regional situation and endangered peace and stability in the Taiwan Strait. We firmly oppose this,” it said in a short statement.

“Theatre forces remain on high alert at all times and resolutely defend national sovereignty and territorial integrity.”

The U.S. Navy’s 7th Fleet said the aircraft had flown in international airspace and that the United States will continue to fly, sail, and operate anywhere international law allows including within the Taiwan Strait.

“By operating within the Taiwan Strait in accordance with international law, the United States upholds the navigational rights and freedoms of all nations. The aircraft’s transit of the Taiwan Strait demonstrates the United States’ commitment to a free and open Indo-Pacific,” it added in a statement.

Taiwan’s defence ministry said the aircraft had flown in a southerly direction through the strait.

Taiwan’s forces tracked the aircraft as if flew through the strait, the ministry said, noting the situation was “as normal”. It did not elaborate.

China, which claims democratically governed Taiwan as its own territory, has stepped up its military activity near the island in the past three years as it seeks to try to force Taiwan to accept Beijing’s sovereignty.

Taiwan’s government says only the island’s people can decide their future and it will not give in to threats.

Washington is Taiwan’s most important international backer and seller of arms despite the absence of formal diplomatic ties, and U.S. support for the island is a constant irritant in Sino-U.S. relations.