The Chinese coast guard said its vessels warned off Japanese ships that "illegally intruded" into waters around disputed East China Sea islets on Tuesday.
The Chinese vessels "took necessary control measures," the coast guard added, without going into details. There was no immediate comment from Japan.
China urged Japan to immediately cease all "illegal activities" in the area and ensure that similar incidents do not recur, the coastguard added.
The waters around Diaoyu Islands, which Japan calls the Senkaku Islands, are claimed by both China and Japan.
The two sides have faced off in the waters, deploying patrol boats and urging the other to leave the area.
