Japan's Defense Ministry says a foreign submarine was sailing underwater in the contiguous zone just outside territorial waters in Okinawa Prefecture on Wednesday and Thursday.
Thursday marked the first known time that a submerged foreign submarine entered the contiguous zone, an area just outside Japanese waters around the Japan-controlled, China-claimed islands.
The Chinese Foreign Ministry retorted by asking Japan to "stop making trouble over the Diaoyu Islands issue and to meet China halfway in accordance with the four principles agreed by both sides in 2014", a spokesman said, referring to a consensus on their differing stances reached by the two countries several years ago. The previous day, Japan lodged a protest with China over the presence of a frigate in the same area. "The presence of the submarine in Japan's contiguous waters, along with that of the frigate, has unilaterally raised tensions".
"Senkaku is Japan's territory and territorial waters internationally and historically and Japan's claim about the contiguous zone is absolutely correct", Defense Minister Itsunori Onodera told reporters Friday morning.
Relations between Japan and China deteriorated in 2012 when Tokyo "nationalised" some of the islets.
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Japan has complained to China after a pair of the country's vessels sailed this week near disputed islands in the East China Sea.
In 2004, the government ordered an SDF maritime security operation after a Chinese nuclear submarine intruded into Japanese waters around the Saki-shima island chain, southern Japan. Japan's actions can in no way change this objective reality. It then re-entered the contiguous zone around Taisho Island, which is part of the Senkakus, before leaving that afternoon.
Japanese defence ministry spokesman Go Yamaguchi told AFP that a submarine of unknown nationality had also been spotted entering the contiguous waters off the disputed islands. It left Thursday afternoon.
Chinese officials defended the move Thursday, saying that the Senkakus are China's territory and that the incident was instigated by two Japanese navy vessels.
The ministry is closely monitoring the submarine's move and collecting information. A Chinese frigate was last seen in the contiguous zone in June 2016.