Japan has lodged an official protest with China after a Chinese naval vessel was spotted in the East China Sea near a group of islands claimed by both countries.
A Chinese frigate was seen Thursday just outside the 22 kilometer limit that makes up Japan's territorial waters surrounding the islands. A spokesman for Japan's Defense Ministry said a foreign submarine, also believed to be Chinese, was detected in the same area Wednesday and Thursday.
In Tokyo, Japanese Vice Foreign Minister Shinsuke Sugiyama summoned Chinese ambassador Cheng Younghua to the Foreign Ministry to express "grave concern" about the incident.
Chinese Foreign Ministry spokesman Lu Kang told reporters in Beijing the two ships were in the area to monitor the activities of Japanese vessels.
Japan and China have been engaged in a long, bitter dispute over the uninhabited islands, called Senkaku by Tokyo and by Beijing as Diaoyu. Japan has controlled the islands since 2012, a move that angered China and led to icy relations between the Asian rivals that have slowly begun to mend.