TOKYO: Japan protested to China on Thursday after spotting a Chinese warship near disputed islands in the East China Sea.

Japanese officials said a Chinese navy frigate was seen off the coast of the Japanese-controlled Senkaku islands, also claimed by China and called the Diaoyu islands. The ship did not violate Japan's territorial waters.

While Chinese coast guard vessels routinely patrol the area, it was the first time a Chinese warship was spotted, officials said.

Chief Cabinet Secretary Yoshihide Suga criticized Beijing for escalating regional tension.

"Japan is extremely concerned about the action that unilaterally escalates tension in the area," Suga said.

The territorial dispute often harms bilateral relations already troubled by wartime history.

Japan is also investigating the presence of three Russian warships in the area about the same time. Suga said officials are analyzing whether the two incidents were coordinated.

Deputy Foreign Minister Akitaka Saiki summoned Chinese Ambassador Cheng Yonghua to the ministry to deliver a protest.

Defense Minister Gen Nakatani, in Singapore to attend an annual international security conference, told NHK national television that the move by Beijing to dispatch a naval ship may just be China's way of stressing its resolve over its territorial claims amid widening international concern over its activities in the South China Sea.