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

Japan's behavior will not change the fact that the Diaoyu Islands and affiliated 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 said at a regular news conference on Thursday.

The Defense Ministry issued a statement saying two Japan Coast Guard vessels entered the northeast contiguous zone of Chiwei Islet, which is a part of the Diaoyu Islands.

Chinese guided-missile frigate Yiyang immediately entered the area to conduct thorough tracking and surveillance of the activities of Japanese forces, and the Japanese vessels have left the area, the statement said.

Japan is well aware of the situation but reversed the truth and hyped the Chinese military's legitimate rights, the statement said, expressing strong dissatisfaction and opposition.

The Chinese military will take all necessary measures to firmly safeguard the territorial sovereignty and security interests of the country, according to the statement.

Japanese Vice-Foreign Minister Shinsuke Sugiyama summoned Chinese Ambassador Cheng Yonghua on Thursday to voice Tokyo's concerns and strongly requested China not to interfere with the flow of improving Japan-China relations, according to AFP.

China urges Japan to stop using the issue to make trouble and take practical actions to improve bilateral relations, Lu said.

China considers its relations with Japan to be important, he said. "We noticed the positive signals from Japan and their high-level officials to improve the relations between the two countries," Lu said.

Japanese Prime Minister Shinzo Abe said last week that he wanted 2018 to be the year in which people in Japan and China recognize "great improvement" in the nations' ties, according to Xinhua.

China hopes Japan will match its words with deeds, work for the same goal with China on the basis of the consensus and spirit of the four-point principled agreement and take practical actions to improve relations, Lu said.