"We can't allow the presence of one single head of state to stymie the world's affairs," Sarkozy told reporters ahead of the Copenhagen conference which opens on December 7.

"The decisive moment is December 17 and 18. If some come at the beginning and others at the end, when will we be able to take decisions?" he asked.

Obama is scheduled to address the meeting in Copenhagen on December 9, the day before he heads to Oslo to receive the Nobel Peace Prize.

The French president urged all heads of state to attend the talks, aimed at hammering out a deal on curbing greenhouse gas emissions to replace the Kyoto treaty when it expires in 2012.

"The commitments must be made at the highest level. Only the heads of state and government can take the major decisions which have to be made," he told journalists after making an extraordinary address to leaders of the 53-nation Commonwealth in Trinidad.

"If some heads of state want to come on the 9th and come back for the moment of decision, that's very positive. But we have agreed: The decisive moment is December 17 and 18," he repeated.

Danish Prime Minister Lars Loekke Rasmussen told Commonwealth leaders Friday that more than 85 heads of state and government have accepted to take part in the Copenhagen talks so far.

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