Three Chinese maritime surveillance vessels entered Japanese territorial waters on Friday near the Senkaku Islands, a Japanese-controlled islet group claimed by China, the Japan Coast Guard said.
Chinese vessels last intruded into Japanese waters on Monday and it was the 44th such incident since last September, when the Japanese government purchased three of the five main islands in the group, which China calls Diaoyu, from a Japanese owner.
The latest intrusion by Chinese ships into the Japanese waters prompted Shinsuke Sugiyama, director general of the Japanese Foreign Ministry's Asian and Oceanian Affairs Bureau, to telephone Han Zhiqiang, China's minister to Japan, to lodge a protest.
The three vessels -- the Haijian 26, 50 and 66 -- entered Japanese territorial waters in the East China Sea from around 2:25 p.m. from southwest of Uotsuri Island in the Senkaku group, according to the 11th Regional Coast Guard Headquarters in Naha, Okinawa Prefecture.
The coast guard said it was warning the vessels to leave the area.
A Chinese fishery surveillance vessel that has been spotted in Japan's contiguous zone, an area outside territorial waters where Japanese law may be applied, since Tuesday night left the zone around 5:30 p.m. Thursday and entered the zone again around 9:15 a.m. Friday, according to the coast guard.