Japan lodged a formal protest with China on Thursday after detecting a Chinese frigate and an unidentified foreign submarine in waters close to the disputed Senkaku Islands in the East China Sea. The incident was the first to involve a submarine in the contiguous zone around the Senkakus.
On Thursday, a foreign submarine was found traveling underwater in the contiguous zone just outside territorial waters near the Senkaku Islands in Okinawa Prefecture.
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 Defense Ministry kept analyzing and tracing the vessel, believing it was a Chinese Navy submarine, as a Chinese frigate was spotted nearby. Japan's Chief Cabinet Secretary Yoshihide Suga warned China "not to undermine the trend of improvement in Japan-China relations".
Japan and China have always been at loggerheads over the tiny, uninhabited islands.
Defense Minister Itsunori Onodera said the missile interceptor system Aegis Ashore, to be deployed in Japan, could be used to intercept Chinese cruise missiles, a statement that may not be well received in Beijing.
Tokyo purchased three of the islets from a private owner in September 2011, sparking anti-Japan protests in dozens of Chinese cities and a boycott of Japanese products. The vessel in that incident was a Type 054 Jiangkai-I-class ship.
China routinely rejects Japanese criticism of such patrols, saying its ships have every right to operate in what China calls its territorial waters.
The last time a foreign submarine was spotted in Japanese contiguous waters was in February 2016.