Japan is "extremely concerned" about China's continued activities around a group of Japan-controlled, China-claimed islands in the East China Sea, the top government spokesman said Friday, ahead of the 10th anniversary of Tokyo putting most of the islets under state control.

Chief Cabinet Secretary Hirokazu Matsuno told reporters the situation has remained "unpredictable" with Chinese coast guard vessels repeatedly entering Japan's territorial waters near the Senkaku Islands since September 2012.

The Senkakus, which China calls Diaoyu, have been a thorny issue in bilateral ties since the Japanese government purchased three of islets, including the biggest island Uotsuri, from a Japanese individual, a decade ago.

File photo taken in September 2012 shows the Senkaku Islands in the East China Sea. (Kyodo)

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Matsuno maintained the government's position that there is no issue of territorial sovereignty to be resolved concerning the uninhabited islands.

"There is no doubt that the Senkaku Islands are clearly an inherent part of the territory of Japan, in light of historical facts and based on international law," said the top government spokesman. "Indeed, they are under the valid control of Japan."

Japan will continue to ramp up its maritime and aerial surveillance activities around the Senkakus, he added.