As campaigners warned that negotiators had five days to avert climate chaos, ministers acknowledged they had to start making progress before the arrival of 120 heads of state for the summit´s climax on Friday.
Sources said the developing countries walked out of working groups at the start of the second week of negotiations here, angered that the conference was weakening in support for the Kyoto Protocol, the core emissions-curbing treaty.
"They have walked out, I am advised, of the working groups," one Western minister told AFP on condition of anonymity.
"This is salvageable. It depends if people want to be constructive."
The move was unleashed by African countries, with the support of the G77 group of developing countries.
They refused to continue negotiations unless talks on a second commitment period to the Kyoto Protocol were given priority over broader discussions on a "long-term vision" for cooperative action on climate change.
The Kyoto Protocol ties the rich countries -- but not developing countries -- that have ratified it to binding emissions curbs.
It does not include the United States, which says the Protocol is unfair as the binding targets do not apply to developing giants that are already huge emitters of greenhouse gases.
"Africa has pulled the emergency cord to avoid a train crash at the end of the week," said Jeremy Hobbs, executive director of Oxfam International.
The walkout delivered another blow to the summit which has already been marred by spats between China and the United States.
A top Western negotiator, speaking on condition of anonymity, said a round-table session of around 50 environment ministers Sunday had been soured by "growing tensions between the Americans and Chinese," saying delegates had merely repeated their previous stances rather than giving ground.
"At the back of everyone´s mind is the fear of a repeat of the awful scenario in The Hague," she told AFP, referring to a climate conference in 2000 on completing the rulebook for the Kyoto Protocol that broke up angrily without agreement.
In an apparent concession, China said it might not take a share of any Western funding for emerging nations to fight climate change.
But in a pointer to the tensions backstage, Vice Foreign Minister He Yafei said China would not be the fall guy if there were a fiasco.
"I know people will say if there is no deal that China is to blame. This is a trick played by the developed countries. They have to look at their own position and can´t use China as an excuse," he told the Financial Times.
Britain´s climate minister, Ed Miliband, urged negotiators to work faster to break the deadlock.
"Leaders are practically on their way ... Leaders always have a very important role in this. But frankly it´s also up to negotiators and ministers not to leave everything up to the leaders, but to get our act together," he said.
Australian Prime Minister Kevin Rudd, whose country is the industrialised world´s biggest per-capita polluter, fretted at the possibility of failure without compromise all round.
"There´s a big risk that we will have conflicting views between developed and developing countries," Rudd said in Australia. "And there is always a risk of failure here."
Campaigners were even blunter, with Greenpeace saying the summit had five days "to avert climate chaos" and that emissions targets so far offered by Western leaders such as US President Barack Obama amounted to "peanuts".
The gathering´s daunting goal is to tame greenhouse gases -- the invisible by-product derived mainly from the burning of coal, oil and gas that traps the Sun´s heat and warms the atmosphere.
Scientists say that without dramatic action within the next decade, Earth will be on course for warming that will inflict drought, flood, storms and rising sea levels, translating into hunger and misery for many millions.
The stakes were underlined when a new UN report said that some 58 million people have been affected by 245 natural calamities so far this year, more than 90 percent of them weather events amplified by climate change.
And a study from the International Union for the Conservation of Nature (IUCN), an intergovernmental group, said climate change threatens the survival of dozens of animal species from the emperor penguin to Australian koalas.
If all goes well, the conference will agree an outline deal of national pledges to curb carbon emissions and set up a mechanism to provide billions of dollars in help for poor countries in the firing line of climate change.
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