Japan lodged a protest Thursday with China after a Chinese marine research vessel entered Japan's exclusive economic zone without prior notice near one of the disputed Senkaku Islands, Foreign Ministry officials said.
Early Thursday morning, the Japan Coast Guard had warned the Chinese ship to halt its operations, with the vessel having been spotted some 65 km northwest of Uotsuri Island, one of the isles claimed by Japan, China and Taiwan.
The ship has been detected inside Japan's EEZ twice so far this month.
On Wednesday, it told the JCG it was planning to continue its research until Aug. 10.
Later Thursday, China defended the vessel's entrance into the EEZ.
The research ship's presence in Japanese waters is "totally legal," Foreign Ministry spokesman Liu Jianchao said at a news briefing in Beijing.
"The Diaoyu Islands ocean territory should not belong to Japan's ocean territory or an economic zone belonging to Japan," Liu said, referring to the island chain by its Chinese name.
Japan has been troubled by a series of incidents involving Chinese ships conducting maritime research in Japan's EEZ without prior notification.
In February 2001, Japan and China agreed on a two-month mutual prior notification system for maritime scientific research in waters between the two countries.