Accusations flew at deadlocked UN climate talks in Lima on Saturday, as the United States warned that failure to compromise could doom the 22-year-old global forum.

Running deep into an unscheduled 13th day, the negotiations got mired in a row between rich and poor countries over sharing out responsibility for cutting climate-altering greenhouse gases.

Nearly a full day after the scheduled close of the meeting, the chairs of a working group tasked with formulating a text presented negotiators with a proposed compromise.

Many rich nations accepted it as a workable blueprint.

But developing countries kicked it out, saying the document failed to balance action on tackling carbon emissions with help for vulnerable economies.

US envoy Todd Stern said the stalemate put an envisioned 2015 climate pact at risk, as well as the credibility of the UN Framework Convention on Climate Change (UNFCCC) -- an offshoot of the 1992 Rio Earth Summit.

"All we have achieved so far will be at risk, and all that we hope to achieve will be at risk as well," Stern told delegates.

"The success of this COP here in Lima is at stake," he said, using the jargon for the UNFCCC's annual Conference of Parties.

"The success of next year's COP in Paris is at stake, and I think the future of the UNFCCC as the body to address climate change effectively at the international level is also at stake."

Facing division, exhaustion and an increasingly fractious mood, the working group handed the baton of seeking a compromise to conference president Manuel Pulgar-Vidal, Peru's environment minister.

"Let us go and work and let us reach a consensus," he begged negotiators.

"It is not time for proposals, it is time for solutions. Let us help each other."

Negotiators in Lima must agree on a formula for guiding a process next year of declaring national carbon-curbing pledges.

These will form the backbone of a worldwide accord to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.

To be sealed in Paris in December 2015 and take effect by 2020, the deal would for the first time bring all 196 UNFCCC members into a single arena for cutting greenhouse gases.

The Lima talks ran into familiar finger-pointing about which countries should do the heavy lifting.

Developing nations insist the West must bear a bigger burden for carbon cuts, having started decades earlier to pollute their way to prosperity.

But rich countries point the finger at developing giants like China and India furiously burning coal to power their rapid growth.

- Anger over adaptation help -

Developing countries also want rich countries' pledges to include commitments to financial help and adaptation aid for shoring up their climate defences.

"Let us not forget millions of poor, because every climate action has a cost," said Indian Environment Minister Prakash Javadekar.

Malaysia complained that appeals for wording on adaptation help had fallen on deaf ears.

"What else do you want us to do? To go on bended knee?" its delegate asked.

And Venezuela objected that there was no reference on the issue of rich-developing country differentiation.

Campaigners in Lima lashed the proposed compromise, saying it had been watered-down.

If approved, the proposal would mean "almost no progress would be made at Lima and all the work punted down the road to be fought over next year in Paris," said Mohamed Adow of Christian Aid.

Asad Rehman of Friends of the Earth said "there would be essentially no outcome for people and the planet. It would be the weakest of weak political statements."

Apart from the pledge format, negotiators must also agree in Lima on a workable negotiating outline for the Paris pact, with just months of haggling time left.

Scientists say the two-degree target is roughly half the warming that can be expected by 2100 on current emissions trends.

But if warming jumps by four degrees Celsius, they say, the world would be a grim place -- gripped by more severe and frequent, droughts, floods, storms and fast-rising sea levels.

Emissions must be slashed by 40-70 percent by 2050 from 2010 levels and to near zero or below by 2100 for a good chance of reaching two-degree warming, the UN's Intergovernmental Panel on Climate Change said in a report issued this year.