Japan has complained to China after a pair of the country's vessels sailed this week near disputed islands in the East China Sea.
A submarine and a Chinese Navy warship have entered the contiguous zone surrounding Japanese territorial waters around the Senkaku Islands in Okinawa Prefecture, the Defense Ministry said Thursday.
China's timing is equally unusual given the fact that Japan is hoping to improve relations between both nations.
Xi Jinping and Shinzo Abe say it's a fresh start for China and Japan.
It was the first time for the government to disclose its detection of a foreign submarine in the contiguous zone near the Senkakus.
Located around 150 km northeast of Taiwan, the uninhabited Senkaku Islands have an area of about 7 sq km and are said to possibly be rich in marine and energy resources. After all, for Japan, the Senkaku area is part of protecting smaller islands and islets throughout the Okinawa region. It then re-entered the contiguous zone around Taisho Island, which is part of the Senkakus, before leaving that afternoon.
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Japan protested to China on Friday after a foreign submarine in waters near disputed islands in the East China Sea was confirmed to be Chinese, calling it a unilateral action and a "serious escalation" that raised bilateral tensions. It left Thursday afternoon. "This unilaterally heightens tensions", Onodera told reporters.
Japanese Prime Minister Shinzo Abe responded to the incidents Thursday by giving instructions to prepare for all possible contingencies and to coordinate closely with the US, according to a statement released by his Cabinet. "China's decision to guard the sovereignty of the Diaoyutai territory is unshaken".
The ministry is closely monitoring the submarine's move and collecting information.
Maritime confrontations and jet interceptions between China and Japan have become commonplace in recent years, especially around the Senkakus.
But China says it is "indisputable" that its navy vessels have every right to operate in what it sees as its territorial waters. A Chinese frigate was last seen in the contiguous zone in June 2016.
"The measures by the Japanese side will not change the established fact that the Diaoyu Islands belong to China, nor will it change China's resolution to safeguard its territory in the Diaoyu Islands", Chinese foreign ministry spokesman Lu Kang said during a regular press briefing on Thursday.