BEIJING, April 26 (Xinhua) -- China on Wednesday reaffirmed its sovereignty over the Diaoyu Island and adjacent islets as well as the Nansha islands, saying it will firmly respond to any challenge to its sovereignty and national security.

Japan released its diplomatic bluebook for 2017 on Tuesday, voicing opposition to the possible unilateral changes of the status quo in the East China Sea and the South China Sea as a result of China's activities there.

"China's activities there are reasonable and legitimate," Foreign Ministry spokesperson Geng Shuang said at a daily press briefing, adding that China has indisputable sovereignty over the Diaoyu Island and its adjacent islets as well as the Nansha islands.

"We will continue to firmly deal with any action challenging China's sovereignty and security," Geng said.

Japan's diplomatic bluebook for 2017 described Japan-China ties as "one of the most important bilateral relationships," saying that Japan stands ready to cement cooperative relationship with China.

Expressing China's willingness to improve bilateral ties, Geng said the onus is on the Japanese side.

Japan should view China's development in an objective and rational manner, adhere to the principles set in the four political documents between the two countries when dealing with differences, and avoid further disturbances to bilateral ties and regional peace and stability, he said.