Two Chinese ships entered waters around Japan-administered islands in the east China Sea early on Monday, the Japanese coastguard said, as tensions mount over the disputed chain.
As of 7:00am (local time), two Chinese maritime surveillance ships were spotted in waters off Kubashima and Uotsurijima, it said, adding that four patrol ships were in "contiguous" waters just outside the territory.
Asia's two largest economies have wrangled about the Japan-administered islands, called Senkaku in Japan and Diaoyu in China, since the 1970s, but the row flared in August after pro-China activists landed on one of them.
Tensions escalated dramatically after the Japanese government bought three of them from their private owners.
The ships' entry came a day after Beijing postponed a ceremony marking the 40th anniversary of the establishment of diplomatic ties with Japan because of the ongoing dispute.
The ceremony was due to take place on Thursday.