Behind the tough words, observers say, Beijing fears such a referendum could set in motion Taiwan's separation from China, and is privately lobbying Washington to do more to pressure Taipei.

Foreign Ministry spokesman Liu Jianchao said on Tuesday that Beijing 'appreciates' repeated US statements against the referendum proposed by Taiwan President Chen Shui-bian and his Democratic Progressive Party.

His comments, reported by Xinhua news agency, came a day after US Deputy Secretary of State John Negroponte issued Washington's sharpest criticism yet of the referendum proposal, calling it a 'mistake' and a step towards formal independence.

Mr Liu said such moves were a threat to cross-strait stability and 'doomed to failure'.

While Beijing seeks at all costs to prevent Taiwan from declaring formal independence, the US occupies a middle ground. Its bottom line is to ensure that war does not break out in the Taiwan Strait.

Beijing passed a law in 2005 sanctioning the use of force against Taiwan should it take 'major actions' that could bring about formal separation from China. A referendum on UN membership, if held, would constitute such a 'major action', mainland scholars have said.

Chinese and US officials have had a flurry of secret meetings in recent weeks aimed at negotiating the 'no-go area' for Taiwan as referendum fever seizes its politicians, Taiwan's United Daily News reported yesterday.

The newspaper said China's top bureaucrat on Taiwan affairs Chen Yunlin is planning a visit to Washington next month. Beijing has not announced the trip.

A mainland scholar told The Straits Times that China cannot act unilaterally on the referendum issue as military threats would only give the corruption-saddled Mr Chen and his party the moral high ground ahead of the island's presidential elections next March.

Said cross-strait expert Zhang Tongxin: 'If China reacts by staging military exercises in the Taiwan Strait, Mr Chen could take the opportunity to declare a state of emergency, push back elections and hold on to his presidency.

'China is stepping up dialogue with the US to rein him in, and I think the US can still do much more.'

clare@sph.com.sg