Beijing on Monday warned Tokyo against any further sovereignty infringement, as Tokyo confirmed its final plan to "nationalize" China's Diaoyu Islands within the month.

Analysts sounded alarm over Japan's accelerating pace of fanning bilateral spats over territorial issues, saying Tokyo's recent messages to Beijing about cooling tensions may be regard as just a "cover" for its true intentions.

Japanese Cabinet Secretary Osamu Fujimura on Monday confirmed that Tokyo is now "in the final stages" of reaching a deal to "buy" part of the Diaoyu Islands from a so-called private owner by the end of September, Kyodo News Agency reported.

Tokyo on Sunday said it has entered the final phase of negotiations, and it will cost Japan 2.05 billion yen ($26.18 million), yet the date of the deal remains unknown.

Fujimura's remark came one day after a survey group arrived in waters near the islands in the East China Sea on Monday morning to conduct an illegal survey.

The survey has prompted a solemn protest from Beijing. "Any unilateral move of the Japanese side over the Diaoyu Islands is illegal and invalid," Foreign Ministry spokesman Hong Lei said on Monday at a daily news conference.

Hong warned that Tokyo's actions, which aim to enhance its illegal rival claim over the islands by "nationalizing" them, will be "in vain".

Liu Jiangyong, an expert on Japanese studies and vice-dean of the Institute of Modern International Relations at Tsinghua University, said the Japanese government has turned a blind eye to the so-called survey on Sunday, which was an intrusion in China's territorial waters and seriously infringed China's sovereignty.

"Tokyo is trying to beef up its reach to the islands by any possible and available approach while trying to avoid major damage to overall China-Japan ties," Liu warned.

Tensions over the issue have escalated since the right-wing Tokyo Governor Shintaro Ishihara in April unleashed his government's plans to "purchase" the islands.

Noda, battling poor domestic public support, announced his cabinet's plan to "nationalize" the islands in early July, a move that prompted further protests from Beijing.

The turbulence occurring in China-Japan relations has "mainly been caused by illegal moves by Japan" that have infringed on China's sovereignty over the Diaoyu Islands, Hong warned.

"In consideration of the big picture of China-Japan ties, China has long called for a steady resolution of the Diaoyu Islands issue through dialogue and negotiations," and he urged Tokyo to return to reconciliation for a resolution.

US Secretary of State Hillary Clinton is arriving in Beijing on Tuesday, and Japanese media said the Diaoyu Islands issue will be on the agenda.

When asked about Washington's position that the islands fall within the scope of the 1960 US-Japan Treaty of Mutual Cooperation and Security, Hong said the treaty is "a relic of the Cold War", and the treaty should not extend beyond Japan and the US or hurt the interests of any third party.

"We hope Washington will refrain from taking a position on the issue in order to maintain regional stability," Hong said.

zhangyunbi@chinadaily.com.cn