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

Japanese officials regularly protest the presence of Chinese coastguard vessels in waters near the islands, but it is the first time since 2018 that a navy ship has been spotted there, according to public broadcaster NHK.

At around 7:44 am on Monday (2244 GMT Sunday), a Chinese navy frigate "was observed entering Japan's contiguous waters" southwest of one of the Tokyo-controlled islands, a statement from the Japanese defence 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 added.

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 defence ministry sources.

The ministry could not immediately confirm the reports to AFP.

In May, Chinese and Russian fighter jets carried out joint flights near the East Asian country as leaders of the so-called Quad bloc -- Japan, United States, Australia and India -- met in Tokyo.

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 Chinese and Russian "annual military cooperation plan".