Two Chinese fishery patrol boats were spotted on Sunday off islands at the centre of a bitter dispute between Beijing and Tokyo, Japan’s coastguard said.
The two vessels began cruising in waters close to the islands, known as Senkaku in Japan and Diaoyu in China, at around 7:40 am (2240 GMT Saturday), a coastguard official said.
"They are still navigating off the Senkaku islands, and our patrol ships and airplanes are warning them by radio not to enter Japanese territorial waters," she said.
Both Tokyo and Beijing claim the potentially resource-rich islands, along with their surrounding waters.
However, Japan has traditionally had more of a presence in the area and administers the islands.
A tense territorial row broke out in September after Japan arrested a Chinese trawler captain after a collision in the area between his boat and Japanese coastguard ships.