"Flying a marine surveillance airplane in airspace above the Diaoyu Islands is completely normal. China urges Japan to stop illegal actions in the waters and airspace of the Diaoyu Islands," Foreign Ministry spokesman Hong Lei said.

Li Guoqiang, deputy director of the Center for Chinese Borderland History and Geography at the Chinese Academy of Social Sciences, said Tokyo's protest and over-hyped media reports confirm its refusal to accept the fact that China has sovereignty over the islands.

Ties between the two countries ran into problems after the Japanese government illegally purchased some of the islands in September.

China has taken a series of countermeasures to assert sovereignty, including constantly patrolling waters off the islands.

Japan has been trying to play up its so-called "actual control" over the Diaoyu Islands airspace to fool the international community, said Wang Xinsheng, a professor of Japanese studies at Peking University.

The incident comes just days ahead of Japan's general election on Sunday.

According to opinion polls, the conservative Liberal Democratic Party is likely to return to power with hawkish former prime minister Shinzo Abe at the helm.

Abe has vowed to take a tough stance over the islands, and he has also promised to boost spending on defense.

However, China's determination to guard the islands will not lessen, Wang Xinsheng said.

"The recent patrols are sending messages that the Japanese should face up to reality,'' Wang said.

US Assistant Secretary of State For East Asian and Pacific Affairs Kurt Campbell reiterated on Thursday that Washington does not take a position on the dispute, but the islands fall within the scope of a 1960 US-Japan security treaty that requires the US military support for Japan.

"We are encouraging all sides to take appropriate steps so that there will be no misunderstanding or miscalculation that could trigger an environment that would be antithetical to peace and stability,'' Campbell told reporters in Malaysia.

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Reuters, Wu Jiao and Liu Yedan contributed to this story.