Chinese government ships were in waters around disputed East China Sea islands on Monday, Japan's coastguard said, as a senior Japanese diplomat prepared for meetings in Beijing aimed at mending frayed ties.
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.
Kyodo News Agency reported that Japan had made a formal protest to China about what it believes is an incursion into its sovereign waters.
It was the latest in a series of incidents at sea that have also included confrontations between warships, with Japan claiming Chinese vessels locked their weapons-targeting radar onto a ship and a helicopter.
Beijing has denied the charge, which rang alarm bells for commentators already warning of the growing possibility of a military exchange that could have disastrous consequences for the region.
The row between Asia's two largest economies blistered in September when Tokyo nationalised three islands in the chain, in what it said was a mere administrative change of ownership.
Months of angry exchanges followed, with the diplomatic temperature rising all the time.
But North Korea's nuclear test last week somewhat dampened the rhetoric, with the international community keen for China to come onboard a broad move to pressure its sometimes-irksome ally.
The Japanese foreign ministry is planning to dispatch Shinsuke Sugiyama, in charge of Asian and Oceanian affairs, on Tuesday for talks with Wu Dawei, China's special representative for Korean Peninsula affairs, local media said.
Sugiyama was also expected to meet Luo Zhaohui, chief of Asian affairs at the Chinese foreign ministry, to discuss Tokyo's concerns about the radar incident.
The Japanese foreign ministry said Sugiyama was going to China for a meeting, but declined to discuss whether he would meet with his Chinese counterparts.