Chinese vessels have entered what Japan considers its territorial waters near disputed islands in the East China Sea.
China's Xinhua news agency confirmed that two civilian surveillance ships were undertaking a "rights defense" patrol near the islands.
The Japanese Foreign Ministry said it had lodged an official protest with the Chinese envoy to Japan against the move.
Relations between Japan and China soured after Japan bought the islands, called Senkaku in Japan and Diaoyu in China.
The purchase sparked anti-Japan protests in cities across China.
The islets are located near rich fishing grounds and potentially huge gas reserves.
Based on reporting by Reuters and AFP