Three Chinese ships entered disputed waters off Tokyo-controlled islands in the East China Sea on Sunday, the Japanese coastguard said, the first such incursion this year.
The Chinese coastguard vessels sailed into the 12-nautical-mile territorial waters at about 8:30 am (2330 GMT Saturday) off one of the Senkaku islands, which China also claims and calls the Diaoyus, Japan's coastguard said.
They left less than two hours later.
Chinese state-owned ships and aircraft have approached the Senkakus on and off to demonstrate Beijing's territorial claims, especially after Japan nationalised some of the islands in September 2012.
Sunday's intrusion was the first since December 29 when three Chinese coastguard ships entered the zone and stayed for around three hours.