Three state-run Maritime Surveillance vessels were clocked in seas off the Tokyo-controlled Senkaku Islands, which Beijing calls the Diaoyus and claims as its own, around 0000 GMT, the coastguard said, AFP reported.
Kyodo News Agency reported that Japan had made a formal protest to China about what it believes is an incursion into its sovereign waters.
The row between Asia's two largest economies blistered in September when Tokyo nationalized three islands in the chain, in what it said was a mere administrative change of ownership.
