China's armed forces on Tuesday vowed unwavering determination and will to guard its territorial sovereignty, in a statement issued by the Defense Ministry.

"We are closely watching the development of the situation, and we reserve the right to take countermeasures," Defense Ministry spokesman Geng Yansheng told reporters on Tuesday.

The Chinese military expressed strong opposition and protest against the Japanese government decision to "nationalize" China's territorial Diaoyu Islands, the spokesman said.

The Diaoyu Islands and affiliated islets have been China's territory since ancient times, this is supported by historical facts and jurisprudential evidence, and the Japanese government's so-called "purchase" of the Diaoyu Island is totally illegal and invalid, Geng said.

"In recent years, Japan has resorted to various excuses for arms expansion, triggered frequent tension in the region, and continually embarked on provocations over the Diaoyu Islands issue, which deserves high alert from its Asian neighbors and the international community," Geng added.