BEIJING, Jan. 11 (Xinhua) -- China's defense ministry has expressed strong discontent with Japanese efforts to sensationalize a legitimate action by the Chinese navy, according to the information office of the ministry Thursday.

The remarks were in response to a claim from Japan that a Chinese vessel had entered the waters of the contiguous zone outside the territorial waters of the Diaoyu Islands.

According to the office, on Thursday morning two vessels of the Japanese maritime self-defense force entered the contiguous zone on the northeast of the Chiwei Islet of China's Diaoyu Islands, to which China's missile frigate Yiyang immediately reacted by tracking and monitoring vessels in the area.

The Japanese vessels have now left the aforementioned contiguous zone, the office said.

"The Diaoyu Island and its affiliated islets have always been an inherent part of Chinese territory, thus the activities of Chinese vessel in the waters are totally legitimate," the office noted. "The Chinese military will continue to firmly defend China's territorial sovereignty and security interests by all means necessary."