COPENHAGEN (AFP) - World leaders work down to the wire on Friday to forge a global climate pact after days of acrimonious negotiations and warnings that failure would have catastrophic consequences for the planet.

Just hours before the finale summit in Copenhagen, leaders and ministers from about 30 countries hammered out the outline of a draft political accord on how to contain global warming and help poor countries cope with the fallout.

Fear of failure has dogged the talks for days as disputes on emissions targets between top polluters China and the United States, procedural wrangling and complaints that poor nations were being sidelined clouded hopes of a deal.

"We tried to find an umbrella political accord, if you like," Swedish Prime Minister Fredrik Reinfeldt, who holds the rotating presidency of the European Union, said after the delegates worked through the night on the draft.

"We tried to advance the main points and have asked the 'sherpas' to work on the text through the night so we can discuss them tomorrow (Friday) morning," he told reporters.

Advisers have resumed work to craft a document that could be presented to US President Barack Obama and another 120 or so world leaders in Copenhagen and then transformed into a binding treaty within six months.

"We've had a very constructive dialogue," said Danish Prime Minister Lars Loekke Rasmussen, but cautioned: "We're not there yet at all."

The declaration will likely call for preventing a rise in global temperatures of more than 2.0 degrees Celsius (3.6 degrees Fahrenheit) compared to pre-industrial times, according to a meeting participant.

Small island nations, their very existence threatened by rising seas, have called for a cap of 1.5 degrees.

The draft will also tally up the pledges from rich nations on cutting greenhouse gases by 2020, and propose a target for all countries by mid-century.

It also outlined a package for poor countries most vulnerable to the ravages of an overheating world, kicking off with 10 billion dollars (seven billion euros) a year from 2010 to 2012, climbing to 50 billion dollars annually by 2015 and 100 billion by 2020.

The text also proposes a range of mechanisms for raising the money, from a tax on air and sea transport fuel to a levy on financial transfers.

After days of deadlock, the mood brightened on Thursday when Secretary of State Hillary Clinton said the United States would contribute to a long-term fund worth 100 billion dollars a year by 2020 to help poor countries fund cleaner technology and shore up defences against the effects of climate change.

The pledge is contingent on an overall deal being secured and the G77 bloc of developing countries said that while the US proposals were "a good signal" they were still not enough.

On Thursday, French President Nicolas Sarkozy joined the doomsayers, warning that the planet's biggest ever meeting on climate faced a looming disaster because of the disputes on emissions cuts.

"There is less than 24 hours. If we carry on like this, it will be a failure," Sarkozy charged. "Failure at Copenhagen would be catastrophic for all of us."

An internal UN memo seen by AFP earlier had shown national pledges for reducing greenhouse gas output would doom the world to warming of up to three degrees Celsius (5.4 degrees Fahrenheit).

Scientists say such a rise would be disastrous, condemning hundreds of millions of people to worsening drought, floods and storms.

"Coming back with an empty agreement would be far worse than coming back empty-handed," said White House spokesman Robert Gibbs.

Clinton accused developing nations -- without naming them -- of backsliding on pledges to open their promised controls on carbon emissions to wide scrutiny, saying the issue is "a deal-breaker for us".

China and India say they are willing to take voluntary measures to slow their surges in heat-trapping greenhouse-gas emissions.

But they are reluctant to accept tough international inspection and insist rich nations shoulder the main burden by accepting huge reduction targets.

"We should not continue to dwell on these issues that are dividing us. We should narrow our differences, otherwise we are facing a failure," Chinese Vice Foreign Minister He Yafei told reporters.

UN chief Ban Ki-moon tried to talk up the prospects of an agreement, saying he had "not seen anything that indicates we cannot seal a deal".

The United States was widely condemned for foot-dragging on climate change under former president George W. Bush, and Obama is hoping his presence will be evidence of a transformation of policy.