The ships were reportedly spotted sailing near the disputed Senkaku Islands in the East China Sea, the media reported, citing the 11th Regional Coast Guard Headquarters based in Naha, Okinawa Prefecture.
The coast guard said that it was the first time a China Coast Guard vessel carrying what appeared to be cannons was found entering Japanese waters.
The Senkaku Islands, known as the Diaoyu in China, are the subject to long-standing regional territorial disputes between China, Japan and Taiwan. The islands are part of Japan's Okinawa Prefecture.
Last year, Japan and China agreed to reduce tensions over the disputed islands, giving way to possible resolution of the stand.
However, Chinese vessels repeatedly intruded Japanese territorial waters around the Senkaku Islands this year. Another Chinese ship was spotted in the area earlier this week.
