China's Foreign Ministry said it had already made "solemn representations" as a survey group from Tokyo wrapped up an illegal survey at the waters off China's Diaoyu Islands on Sunday.

"The Chinese side reiterates that any unilateral action by the Japanese side on the Diaoyu Islands is illegal, invalid and will not change the reality that the Diaoyu Islands and its affiliated islands belong to Chinese territory," it said in a statement.

Analysts issued warnings to watch out for Tokyo's duplicitous dirty tricks, as Japan is enhancing its rival claim over the islands while calling for cooling tensions with China.