The European Union said key developing states backed its roadmap for a binding pact to fight global warming, but warned UN climate talks could still collapse on Friday unless all major polluters came on board.
The EU plan sets a 2015 target date for a new deal that would impose binding cuts on the world's biggest emitters of the heat-trapping gases, a pact that would come into force up to five years later.
EU Climate Commissioner Connie Hedegaard said Brazil and South Africa, whose growing economies are heavy polluters, now supported binding cuts to emissions of the greenhouse gases that cause rising sea-levels and increasingly extreme weather.
"The success or failure of Durban hangs on a small number of countries who have not yet committed to the (EU) roadmap and the meaningful content it must have," Hedegaard said. "If there is no further movement from what I have seen until 4 o'clock this morning, I don't think there will be a deal in Durban. That's what we are faced with."
The EU plan envisages a new deal reached by 2015, and put into effect by 2020, imposing binding cuts on the world's biggest emitters of the heat-trapping gases.
The EU is trying to forge a coalition of the willing behind its plan in order to put pressure on the world's top three carbon emitters - China, the United States and India - to sign up. None are bound by the Kyoto Protocol, the only global pact that enforces carbon cuts.
Washington says it will only pledge binding cuts if all major polluters make comparable commitments. China and India say it would be unfair to demand they make the same level of cuts as the developed world, which caused most of the pollution responsible for global warming.
Many envoys believe two weeks of climate talks in the South African city of Durban will at best produce a political agreement, with states promising to start talks on a new regime of binding cuts in greenhouse gases. Anything less would be disastrous, they say.
"We have never seen progress in these climate negotiations unless there has been an alliance between developed and developing countries," said Alden Meyer of the Union of Concerned Scientists. "The EU acknowledged positive movement from South Africa and Brazil and is now trying to get the US, India and China on board," he said.
"People see where the deal could be in Durban but the question is whether people want to be part of that," the EU's Hedegaard said.
If the discussions hold to form, envoys will extend discussions and release their decisions on Saturday.
Three UN reports released in the last month show time is running out to achieve change. They show a warming planet will amplify droughts and floods, increase crop failures and raise sea levels to the point where several island states are threatened with extinction.
South African President Jacob Zuma has said Durban will be a failure if a Green Climate Fund, designed to help poor nations tackle global warming and nudge them towards a new global effort to fight climate change, is not put into force.
A group of 48 of the least developed countries has said it backed the European plan for a firm timetable, joining 43 small island states. Japan has said it shares "common ground" with Europe while Canada and several other developed countries have shown their support.