Two China Coast Guard ships entered Japanese territorial waters around the Senkaku Islands in the East China Sea on Tuesday, the Japan Coast Guard said.
A Japan Coast Guard patrol ship observed them for about two hours before they left the area, officials said.
The Haijing 2166 and the Haijing 2401 entered Japanese waters southwest of Uotsuri in the Senkaku chain around 10 a.m., according to the 11th Regional Coast Guard Headquarters in Naha, Okinawa Prefecture.
The Chinese vessels left the waters southeast of Minamikojima, another island in the Senkaku chain, shortly past noon, the Japan Coast Guard said.
China Coast Guard ships last entered territorial waters in the area on Friday. Chinese government vessels have been recorded intruding on 31 days so far this year.
China contests Japan's sovereignty over the uninhabited islets. Its ships have repeatedly entered the waters around them in recent years, raising tensions between the two countries.