Four Chinese government ships were in territorial waters around disputed islands in the East China Sea on Tuesday, the Japanese coastguard said.
The four maritime surveillance ships entered the waters shortly after 12.30pm (0330 GMT), the coastguard said in a statement, adding that it was
telling the ships to leave the area.
"Patrol ships from our agency have been telling them to sail outside of our territorial waters. There has not been any response" from the Chinese ships, the Japan coast guard said.
Two other Chinese official ships were sailing near the island chain, but not in what Japan claims as its territorial waters, the agency also reported in a separate statement.
It was the first time in about a week that Chinese ships had entered the waters, and followed a lull in a vitriolic diplomatic spat over the sovereignty of the islands known as Senkaku in Japan and Diaoyu in China.
Typhoon Jelewat passed through the area over the weekend, but weather was calmer on Tuesday.