Four Chinese coast guard ships tried to approach a Japanese-registered private vessel in Japanese territorial waters around the Senkaku Islands in the East China Sea on Monday.

Japan Coast Guard patrol ships secured the safety of the 997-ton Shinsei Maru and warned the Chinese ships to leave the waters.

The intrusion by Chinese government ships into Japanese waters around the Senkaku chain was the second this year, following one on Jan. 10. The Japanese-administered islands in Okinawa Prefecture are claimed by China, which calls them Diaoyu.

According to the Japan Coast Guard, the four Haijing ships entered Japanese waters between around 2:45 a.m. and 6:05 a.m. and tried to approach the Shinsei Maru and three fishing boats. The Chinese ships left the area between around 12:35 p.m. and 1:50 p.m.

The city government of Ishigaki in Okinawa has rented the Shinsei Maru for marine research activities around the Senkakus.

China Marine Police spokesperson Gan Yu said the Shinsei Maru and four other Japanese ships illegally entered the territorial waters of the Diaoyu islands before being driven away by Chinese coast guard vessels, Chinese state broadcaster CCTV reported.

The disputed East China Sea islets have long been a sticking point in bilateral relations.

"We urge the Japanese side to immediately stop all illegal activities in these waters and ensure that similar incidents will not happen again," Gan was quoted as saying.