On Thursday, the Japanese Ministry of Defense said that a Chinese frigate and a submarine of unknown origin were both spotted in the contiguous zone around the disputed Senkaku/Diaoyu Islands in the East China Sea.
While incursions into Japan's contiguous zone and territorial sea around the Senkakus are fairly common for Chinese maritime law enforcement vessels and civilian fishing trawlers, incursions by Chinese naval vessels are comparatively rarer. The first was in June 2016.
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.
Japan controls the uninhabited Senkaku islands, which China also claims under the name Diaoyu islands.
According to the Japanese Defense Ministry, the submarine was first detected Wednesday afternoon in a contiguous zone near Miyako Island elsewhere in Okinawa Prefecture. The sub exited the contiguous zone and headed toward the Senkakus on Thursday morning. The Japanese government maintains that the islands are an inherent part of Japan's territory.
Chief Cabinet Secretary Yoshihide Suga told reporters Thursday that Japan will "definitely defend our territory, territorial waters and airspace".
Japanese Vice Foreign Minister Shinsuke Sugiyama summoned Ambassador to Japan Cheng Yonghua to the Foreign Ministry and protested the frigate's entry into the zone, expressing "grave concern".
Although often strained over territorial and historical grievances, bilateral ties have recently shown signs of a thaw, with Japanese Prime Minister Shinzo Abe and Chinese President Xi Jinping agreeing late a year ago to make a "new start".
The mechanism has been subject to a decade of negotiations. "China's decision to guard the sovereignty of the [Diaoyu] territory is unshaken". Officials from both sides met in Shanghai for two days and chose to implement the hotline.
Relations between Japan and China deteriorated in 2012 after Japan bought the disputed islands from a private owner. It does not include the contiguous zone, where a coastal state is afforded some law enforcement rights under global law.
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.