Four Chinese government ships have entered territorial waters around disputed islands in the East China Sea, the Japanese coastguard said.
The islands - known as Senkaku in Japan, Diaoyu in China, and Tiaoyutai in Taiwan, are claimed by all three countries.
The four maritime surveillance ships entered the waters shortly after 12:30pm local time (03:30 GMT) on Tuesday, the coastguard said, which told the ships to leave the area.
"Patrol ships from our agency have been telling them to sail outside of our territorial waters," said the coastguard. However, it added that "there has not been any response" from the Chinese ships.
The coastguard reported that two other official Chinese official ships were sailing near the island chain, but not in what Japan claimed as its territorial waters.
It was the first time in about a week that Chinese ships had entered the waters, after a lull in the diplomatic dispute over the islands' sovereignty.
Despite warnings from Japan's well-equipped coastguard, official Chinese vessels had frequently sailed into the waters until last Monday.
Koichiro Gemba, Japan's foreign minister, said the Japanese government filed an immediate protest with China over the latest case, telling reporters: "We want the Chinese side to exercise self-restraint."
The disputed islands are surrounded by rich fishing grounds, and the seabed is believed to contain rich mineral reserves.