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.

The survey group, organized by Tokyo's prefectural government, was comprised of 25 people, including officials and estate valuers, to do surveys at the waters near the islands, the Kyodo News Agency reported.

Tokyo's prefectural government is also preparing another survey on the islands in October. Right-wing Tokyo Governor Shintaro Ishihara has vowed to take part in the trip despite possible disapproval from the central government.

Japanese Prime Minister Yoshihiko Noda announced his cabinet's plan to "nationalize" the islands in early July, a move that prompted protests.

Tokyo on Sunday said it has entered the final phase of negotiations with the islands' "private owner", and it is estimated to cost Japan 2 billion yen ($25.54 million), Japanese newspaper Mainichi Shimbun reported.

But the date for the deal is still "under consideration", and Tokyo indicated that final plans may be released after Noda returned from the upcoming Asia-Pacific Economic Cooperation leadership summit, which is scheduled to start on Saturday, Mainichi said.

On Friday, Tsuyoshi Yamaguchi, visiting Japan's parliamentary senior deputy minister of foreign affairs, handed to State Councilor Dai Bingguo a letter from Noda to President Hu Jintao.

Meanwhile, Chen Guangbiao, a Chinese billionaire and high-profile philanthropist entrepreneur, unleashed his latest plan to publicize China's sovereignty over the islands through Japanese media.

Chen has contacted some Japanese media about publicizing the issue, and negotiations have been "tough", he told the Beijing Youth Daily.

An unnamed influential Japanese newspaper has preliminarily agreed, Chen said.

Zhang Yuqiang, a researcher on international communication studies at Communication University of China, said "as China speeds up its pace to integrate itself into the world, Chinese citizens are showing growing visions and willingness to safeguard China's sovereignty".

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