Japan has filed an official protest against China after a Chinese naval ship was spotted in the East China Sea near a group of islands claimed by both countries.
A Chinese frigate was seen Thursday beyond the 22 kilometer formed a territorial waters of Japan that surrounds the islands.
A spokesman for the Japanese Ministry of Defense said a foreign submarine, also believed to belong to China, was detected in the same region on Wednesday and Thursday.
In Tokyo, Japanese Foreign Minister Shinsuke Sugiyama summoned Chinese ambassador Cheng Younghua to the Foreign Ministry to express "serious concerns" about the incident.
Chinese Foreign Ministry spokesman Lu Kang told reporters in Beijing that the two ships are in the region it is to monitor the activities of Japanese ships. [lt]