China Coast Guard vessels have been asserting Beijing’s claims to waters within Japan’s exclusive economic zone (EEZ) south of Okinawa Prefecture’s far-flung Yonaguni Island and east of Taiwan, Tokyo said Monday.

The unusual move by Beijing follows an agreement between Japan and the Philippines last month to launch maritime border delimitation talks, which overlap with areas claimed by China — a move that has angered the Asian powerhouse and prompted it to conduct special “law enforcement” patrols in the area in response.

“We have confirmed that China Coast Guard ships have been intermittently navigating within Japan’s exclusive economic zone south of Yonaguni Island and have made unilateral claims to the waters,” Chief Cabinet Secretary Minoru Kihara told a news conference Monday.

Kihara, the government’s top spokesperson, called the moves “unacceptable” and said Tokyo “has repeatedly lodged protests with the Chinese side through diplomatic channels.”

At a summit late last month, Prime Minister Sanae Takaichi and Philippine President Ferdinand Marcos Jr. agreed that their countries would ​begin formal talks on delimiting the maritime boundary of the EEZ ‌and the continental shelf between the two countries “in ​accordance with international law.”

Although details of the discussions remain scant, the Chinese Foreign Ministry has said that the envisioned area covers waters east of Taiwan, ⁠which Beijing claims as its own territory, adding that such talks are “completely illegal, null ⁠and void.”

China Coast Guard ships have ramped up their activities in the area, with Kyodo News reporting Sunday that the vessels have begun escorting the country’s public vessels in the EEZ, according to an analysis of ship-tracking data.

Earlier this month, NHK reported that two China Coast Guard ships navigating in the EEZ had for the first time claimed the waters are under China’s jurisdiction, delivering their message to the Japan Coast Guard via radio.