Sunday marked 10 years since Japan's move to put the China-claimed Senkaku Islands under state control, and comes at a time when Tokyo says it is "extremely concerned" over Beijing's ongoing activities around the contested islets.

The Senkakus, called Diaoyu by China, have been problematic for bilateral ties since the Japanese government purchased three of the islets a decade ago, including the biggest island Uotsuri, from a Japanese individual.

File photo taken in June 2011 shows the contested Senkaku Islands at the center of a diplomatic disagreement between Japan and China. (Kyodo)

China began claiming the Japan-administered uninhabited islets in the East China Sea in the early 1970s after United Nations studies indicated potentially lucrative gas reserves might be located around them.

In the intervening years since Japan decided to nationalize the islets on Sept. 11, 2012, Chinese coast guard vessels have regularly entered territorial waters around the Senkakus, with the Japan Coast Guard saying the incursions occurred on 40 days in 2021, a total exceeded only by that of 2013.

As of the end of August, the number has reached 25 days so far this year, the Japanese coast guard said. It also said the contiguous zone around the islets has been breached by Chinese naval vessels on four known occasions since June 2016, including an incident in July this year.

Japan's Chief Cabinet Secretary Hirokazu Matsuno told a press conference Friday that the situation remains "unpredictable" and that the country is "extremely concerned" by China's ongoing activities in the waters around the islets.

"That the Senkakus are our nation's inherent territory is without doubt historically and based on international law," Foreign Minister Yoshimasa Hayashi told a separate news conference Friday. He emphasized that the situation "affords no room for complacency, and we are deeply concerned."

Behind the heightened tensions is China's increasing militaristic pressure on Taiwan, only around 170 kilometers from the islets, and fears a clash with the United States could lead to war reaching Japan's southwestern Nansei Islands chain, including the Senkakus.

But the Japanese and Chinese sides are looking for options to stabilize relations as the countries mark 50 years of normalized relations on Sept. 29. Following high-level talks in August, plans are being considered for both nations' foreign ministers to meet in New York this month.

A meeting between Japanese Prime Minister Fumio Kishida and Chinese President Xi Jinping is also eyed for this year. But Beijing has not softened its stance on the islets, and there is currently no expectation of a resolution to the dispute.

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