Chinese ships were spotted near a group of Japan-controlled islands for the 100th straight day on Wednesday, the longest streak since Tokyo put them under state control in 2012.

Japan said China's repeated dispatch of vessels to the Senkaku Islands in the East China Sea is an "extremely serious" issue, prompting Tokyo to beef up patrols and lodge protests with Beijing.

During the 100-day period from April 14, Chinese ships were spotted in Japan's territorial waters around the uninhabited islands on 11 days, according to officials.