Delegates at a UN climate conference in Canada have reportedly agreed on talks to cut greenhouse gases after 2012, but it is unclear if the US is included.
The heavily industrialised US is sensitive to any energy controls
Sources close to the negotiations in Montreal said an informal agreement had been reached on the last day to plan long-term action on carbon emissions.
The Bush administration is wary of new commitments. It rejected Kyoto in 2001, saying it would damage the US economy.
Former US President Bill Clinton has made a strong plea for acceptance.
Addressing the conference at the invitation of the city of Montreal, he said to loud applause that there should be a "serious commitment to a clean-energy future".
If existing clean energy and energy conservation technologies were applied in full, Mr Clinton said, the US could "meet and surpass the Kyoto targets easily in a way that would strengthen, not weaken, [its] economy".
Global warming and melting ice, he suggested, could lead to a future climate conference in Canada being held on "a raft somewhere".
Last week delegates finalised a rule book for Kyoto, formally making it fully operational after years of negotiation and ratification.
The 1997 treaty commits industrialised countries to cut their combined carbon emissions to 5% below 1990 levels by the years 2008 - 2012.
The Montreal agreement would give the 157 Kyoto signatory-states seven years to negotiate and ratify new measures.
Delegates talked until 0630 (1130 GMT) on Friday and the closing session began just a few hours later.
'A duck is a duck'
The all-night haggling led to the chairman of the talks, Canadian Environment Minister Stephane Dion, producing a bland text, the BBC's Elizabeth Blunt reports from Montreal.
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The discussion and workshops he is proposing are "without prejudice to any future negotiations, commitments, process, framework or mandate", additions to the text say.
But US lead negotiator Harlan Watson is reported to have told the Canadians "If it walks like a duck and quacks like a duck, it's a duck" before walking out of the overnight talks.
In other words, our correspondent says, they feel they are being lured into negotiations on future commitments and they want nothing to do with it.
The US has formally rejected Canadian Prime Minister Paul Martin's suggestion that it needed to heed "the conscience of the world".
"If you want to talk about global consciousness, I'd say there's one country that is focused on action... dialogue... cooperation and... helping the developing world, and that's the United States," said state department spokesman Adam Ereli in Washington.
As reports emerged of Mr Watson's duck remarks, environmentalists began buying up toy ducks in Montreal shops to hand them out at the conference, Reuters news agency reports.
Jennifer Morgan, climate-change expert for environmental group WWF, said that Mr Watson's decision to leave the talks overnight showed "just how willing the US administration is to walk away from a healthy planet and its responsibilities".
The plans for a post-Kyoto dialogue have also been challenged by Saudi Arabia, which wants more action to compensate petroleum-exporting countries standing to lose revenue from alternative energies.
Japanese Environment Minister Yuriko Koike called on developing countries like India and China, which are exempt from Kyoto, to join in the fight against global warming.
Indian negotiator Andimuthu Raja said growth and the elimination of poverty must take precedence over mitigating the effects of climate change.