China said the mission in its own territorial waters is beyond reproach and justifiable after Japan lodged a protest for a Chinese frigate entering into waters surrounding the Diaoyu Islands on Thursday.

Japan's behavior will not change the fact that the Diaoyu Islands belong to China, and it can hardly shake the firmness and determination of China in safeguarding its territorial sovereignty, Foreign Ministry spokesman Lu Kang told a regular news conference on Thursday.

According to him, two Japanese coast guard vessels entered the waters surrounding the Diaoyu Islands, and Chinese naval vessels conducted surveillance over the activities of the Japanese side, he said.