Todd Stern, the US president's envoy for climate change, said the European Union was the only remaining "major player" that would potentially support a continuation of the protocol after its provisions expire in 2012. The lack of support from other countries bodes ill for the forthcoming talks at Durban.
The Kyoto Protocol is an international agreement that imposes limits on the greenhouse gas emissions of some signee countries that was negotiated in the Japanese city of Kyoto in 1997.
Kyoto is the only treaty which binds nearly all of the world's industrialised countries to cut their greenhouse gas emissions but Stern cast doubt on its future.
"Of the major players in the Kyoto Protocol, my sense is that the EU is the only one still considering signing up in some fashion to a second commitment period. Japan is clearly not, Russia is not, Canada is not and Australia appears unlikely."
His words were the broadest hint yet by one of the most influential figures at the talks that the negotiations may stall unless the Kyoto protocol is dropped.
Behind the scenes, many experts are advising that arguments over the future of the protocol are likely to be fruitless. Sir David King, the British government's former chief scientific advisor, told the Guardian recently that the protocol should be dropped, as it was only an impediment to reaching a new international agreement on averting global warming.
The future of the protocol is a key question at the United Nations climate negotiations, because most big developing countries have stipulated that the 1997 treaty must be continued as a condition of any future climate change agreement.
Those developing countries are furious that rich countries are thinking of dumping the hard-fought protocol, which they insist must be the foundation of any future agreement.
Disagreement between developed and developing countries on whether to ditch the protocol was one of the biggest reasons why the Copenhagen climate summit in 2009 failed to reach a clear conclusion.
The US does not take part in Kyoto Protocol discussions, because it has never ratified the treaty and the current administration has followed its predecessor in vowing not to do so.
However, as the chief climate change negotiator for the world's biggest economy, Stern's views carry enormous weight in the debate on possible future international measures.
"The Kyoto Protocol is one of the toughest if not the toughest part of the negotiations," Stern admitted. "The US is not part [of those discussions] but what happens to [the protocol] is relevant to whether there will be understandings on future regimes [and these] are still controversial and difficult subjects."
He said that the US had participated, in recent days, in international "conversations about future regimes" on controlling greenhouse gas emissions, and the question of whether there should be a single global regime on cutting emissions or one that could run concurrently with a continuation of the Kyoto Protocol. Other issues discussed included whether any future regime should be legally binding.
However, Stern said there had been no discussions on trying to find a way forward among a smaller number of countries, outside the UN process. Some countries have privately criticised the UN process for the unwieldy and bureaucratic nature of its negotiations.
Stern warned that the US would not countenance a new climate regime that contained "escape hatches" for some countries, and hinted that countries now labelled as "developing" should be drawn into taking on obligations on emissions.
"It could not be on the basis of categories of countries that were articulated in 1992 [when the parent treaty to the Kyoto Protocol was signed]," he said. In that parent treaty, the UN Framework Convention on Climate Change, to which the US is a signatory, economies such as China, India and Brazil were judged to be developing and thus escaped obligations to cut their emissions.
However, the rapid growth of these economies in the past two decades has changed the international scene, in the US view.
Stern's words were an indication that big emerging economies such as China – the world's second biggest economy by output – must take on legally binding obligations if the US were also to consider doing so.
At the negotiations in Copenhagen and last year in Cancun, China, India and a few other big emerging economies agreed to curbs on the future growth of their emissions but fell short of pledging absolute reductions, and the resulting agreements do not have the legal status of a fully articulated treaty like the Kyoto Protocol.
Stern insisted that he was "not pessimistic" about the prospect of important progress being made at Durban towards a new international agreement on climate.
No one expects that any significant new agreement will be signed this year. There will be another, bigger conference at the end of next year in Rio de Janeiro.
Fiona Harvey for The Guardian