Three Chinese government ships entered the waters of disputed islands on Friday, Japan's coastguard said, more than a year after the then-Tokyo governor set off the row by announcing plans to buy them.
The Chinese maritime surveillance vessels were spotted off the Senkaku islands, which China calls the Diaoyus, in the East China Sea at around 2:30 pm (0530 GMT), the coastguard said.
It is the latest episode in a fraught few months that has seen repeated stand-offs between official ships from both sides as they have jostled over ownership of strategically-important and resource-rich islands.
The territorial row blistered in September when Tokyo nationalised three islands in the chain, in what it said was a mere administrative change of ownership and one intended to pre-empt a more volatile purchase by nationalist Tokyo governor Shintaro Ishihara.
Tokyo's move prompted angry anti-Japan demonstrations across China, which has intensified claims to the islands it says should have been "returned" in the post-World War II settlement Tokyo made.