But Washington can still attend talks about extending the treaty after 2012.

"The United States is having an easy ride. I am concerned about the situation," said Michael Zammit Cutajar, appointed this month to head UN talks to extend the treaty beyond 2012.

He said Washington was "off the hook" by being able to monitor meetings but not having to undertake any commitments.

Any chance of getting a commitment from Washington was unlikely before President George W. Bush stepped down in January 2009, he added.

"What happens in the United States is the big question. Post-2008 is the year you have to look at."

The present pact commits 35 rich nations, including the European Union, Japan and Canada, to cut emissions by at least 5.2 percent below 1990 levels by 2008-12. Any new caps would also be limited to industrialised countries.

Bush says that investments in new technologies, such as hydrogen or solar power, are better than Kyoto's binding caps.

The United States is a signatory to Kyoto's umbrella treaty -- the 1992 United Nations Framework Convention on Climate Change -- entitling it to a seat as an observer at the Kyoto talks.

Cutajar said he was also concentrating on keeping on side existing Kyoto ratifiers -- countries such as Canada and Japan -- seen wavering as they are way behind existing targets.

He said he stressed fairness and a scientific and economic basis for new caps.

"Fairness is a very important political concept. The new commitments have to be seen to be fair in relation to competitors and even in relation to the big emerging economies."

But he acknowledged that there was no prospect of imposing emission caps on China -- a sore point, given its burgeoning economy is increasingly a competitor to the rich nations which face caps under the treaty.

China could face voluntary measures instead.