Four Chinese government ships were sailing in the territorial waters of disputed Tokyo-controlled islands on Tuesday, the coastguard said, the day after Japan's premier promised to beef up security.
The maritime surveillance ships entered the 12-nautical-mile zone of territorial waters around one of the East China Sea islands at around 0200 GMT, the coastguard said.
Separately, two fisheries patrol ships were spotted in the so-called contiguous waters, which extend a further 12 nautical miles, of another island in the chain, coastguards said.
The latest move came after Japanese Prime Minister Yoshihiko Noda said Monday Tokyo would "strengthen security" around its coast.
Chinese vessels have moved in and out of what Japan says is its sovereign territory over the last nearly two months since Tokyo nationalised some of the islands, known as the Senkakus in Japan and Diaoyus in China.
As well as the potential mineral reserves to which ownership of the islands grants access, both countries have considerable amounts of national pride at stake in the decades-old spat.
The dispute has hit the huge trade relationship between the two largest economies in the region. Senior representatives from both governments are reportedly readying for a third round of talks on the issue.