The Senkakus/Diaoyu Islands in the East China Sea, September 2012 (Photo: Kyodo)

The Japan Coast Guard said four Chinese Coast Guard ships were spotted in waters around the Senkaku, one of them apparently armed with a machine gun.

Japan said China's repeated dispatch of vessels to the Senkakus/Diaoyu is "extremely serious" and has prompted Japan to beef up patrols and to lodge formal protests with Beijing.