Four China Coast Guard ships entered Japanese waters around the Senkaku Islands on Sunday morning, the Japan Coast Guard said.
The vessels, including one equipped with what appeared to be machine guns, entered the waters around the uninhabited islets in the East China Sea at around 9:50 a.m. and left about 90 minutes later after warned off by the Japan Coast Guard.
The intrusion was the first of the year for the China Coast Guard. Chinese vessels last entered Japanese waters on Dec. 30.
China has stepped up its claim to the uninhabited islets since the Japanese government purchased most of them from a private Japanese owner and effectively nationalized them in 2012.