Chinese fighter planes kept an eye on a US Navy patrol plane flying through the crucial Taiwan Strait as China conducted a third day of military exercises to the south of the island Beijing considers to be its sovereign territory.

China has been enraged by US military deployments in the narrow strait, most often of warships but also of planes, claiming that China “has sovereignty, sovereign rights, and jurisdiction” over the waterway. Taiwan and the United States both argue that it is an international waterway. The P-8A Poseidon maritime patrol and reconnaissance jet, which is also employed for anti-submarine missions, sailed over the strait in international airspace, according to the US Navy’s 7th Fleet.

“By operating within the Taiwan Strait in accordance with international law, the United States upholds the navigational rights and freedoms of all nations,” it said in a statement.

China’s military described the flight as “public hype”, adding it sent fighters to monitor and warn the U.S. plane.

“Troops in the theatre are always on high alert and will resolutely defend national sovereignty and security as well as regional peace and stability,” the People’s Liberation Army’s Eastern Theatre Command said in a statement on its WeChat account.

Taiwan’s defence ministry said Chinese warplanes and warships carried out a third day of exercises to the island’s south on Thursday, and that it detected 26 aircraft including advanced Chinese J-16 and Su-30 fighters flying out to sea and “responding to” the U.S. Poseidon.

The ministry said the U.S. aircraft had stuck to the strait’s median line and flew in a southerly direction on Thursday morning, and that Taiwan’s forces kept watch.

The median line normally serves as an unofficial barrier between Taiwan and China.

However, since last August when China held large-scale war games around Taiwan, Chinese military aircraft have been frequently crossing the line, though generally quite briefly.

China’s latest drills near Taiwan have involved fighters, bombers and warships, with the aircraft mainly flying to the island’s south and out into the Pacific through the Bashi Channel that separates Taiwan from the Philippines, according to maps provided by Taiwan’s defence ministry.

China has not commented on the exercises, which have taken place less than two weeks before Taiwan stages its own annual drills and as NATO alliance leaders said China challenges its interests, security and values with its “ambitions and coercive policies”.