Three Chinese government ships entered the waters of disputed Tokyo-controlled islands on Monday, Japan's coastguard said, as a long-running row shows no sign of fading.
Maritime surveillance vessels were spotted in the 12-nautical-mile zone off the Senkaku islands, which China calls the Diaoyus, in the East China Sea shortly after 9am (0000 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.
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.
In one of the more intense incidents, Chinese warships locked their weapons-targeting radar on a Japanese destroyer, and opposing fighter planes have shadowed each other on numerous occasions amid warnings a slip-up could lead to a military showdown.
In late April, eight Chinese government vessels sailed into the disputed waters and Japanese Prime Minister Shinzo Abe vowed he would "expel by force" any Chinese landing on the islands.