TOKYO: Japan lodged a protest with China on Monday over a Chinese naval vessel sailing near disputed islands, a Japanese official said, as reports emerged about Russia also sending its own navy ship to the area.

The islets in the East China Sea, known as the Senkaku by Tokyo and the Diaoyu by Beijing, are at the center of a long-running dispute between Japan and China.

Japanese officials regularly protest the presence of Chinese coast guard vessels in the waters near the islands, but it is the first time since 2018 that a navy ship has been spotted there, according to public broadcaster Nippon Hoso Kyoka (NHK or Japan Broadcasting Corp.).

At about 7:44 a.m. on Monday (6:44 a.m. in Manila), a Chinese navy frigate “was observed entering Japan’s contiguous waters” southwest of one of the Tokyo-controlled islands, a statement from the Japanese defense ministry said.

Contiguous waters are a 12-nautical-mile band that extends beyond territorial waters.

“We expressed grave concerns and lodged our protest to the Chinese side through a diplomatic route, and urged them to prevent a repeat” of similar incidents, Deputy Chief Cabinet Secretary Seiji Kihara told reporters.

The islets “are Japanese territory from the viewpoints of both history and international law,” he said.

Separately, a Russian naval ship was also spotted in the contiguous waters of the disputed islands on Monday morning, NHK, Jiji Press and other Japanese media reported, citing anonymous defense ministry sources.

The ministry could not immediately confirm the reports to Agence France-Presse (AFP).

The protest comes after Chinese and Russian fighter jets jointly flew near the East Asian country in late May as leaders of the so-called Quad bloc — Japanese Prime Minister Fumio Kishida, United States President Joe Biden, Australian premier Anthony Albanese and Indian Prime Minister Narendra Modi — met in the Japanese capital.

While the planes did not breach territorial airspace, Japan said the move was “provocative,” given that the timing coincided with the leaders’ summit.

Beijing said the flights were part of the Chinese and Russian “annual military cooperation plan.”