Four Chinese government ships entered disputed waters in the East Asia Sea at the centre of a bitter row with Tokyo on Saturday, the Japanese coastguard said.
"We are telling them to leave the area," a coastguard spokesman told AFP, after the ships sailed into waters around the Senkaku islands -- known in Chinese as the Diaoyu islands -- shortly after 9:00 am (0000 GMT).
The ships were among five vessels that have been sailing in and out of the disputed zone since last week.
The Japanese foreign ministry summoned Beijing's envoy on Thursday after the ships spent more than a day in Japanese territorial waters, marking their longest such incursion since the long-simmering dispute erupted again last year.
The islands are regarded as a potential flashpoint that some observers fear could lead to armed conflict between the two nations.
The East China Sea archipelago is located in rich fishing grounds and is believed to harbour vast natural resources below its seabed.
The long-running dispute flared after Japan nationalised some of the disputed chain in September, setting off a diplomatic row and riots across China.
A Chinese boycott of Japanese brands quickly followed and the issue has all but frozen relations between the two Asian giants.