Japan's Coast Guard says 4 Chinese patrol ships temporarily entered Japanese territorial waters near the Senkaku Islands in the East China Sea on Friday.
Coast Guard officials said the ships breached the territorial waters off Uotsuri Island in Okinawa Prefecture shortly after 10 AM.
Foreign Ministry officials lodged a protest with the Chinese Embassy in Tokyo. They said intrusions into Japan's waters are an infringement of its sovereignty, and demanded that the vessels leave the area at once.
The 4 ships stayed in Japan's waters for about 90 minutes before moving away.
Japan controls the Senkaku Islands.
The Japanese government maintains the islands are an inherent part of Japan's territory.
China and Taiwan claim them.