Big emitters are backing a deal for rich countries to slash emissions by 80 per cent by 2050 under a "Copenhagen accord" that is being hammered out in the dying minutes of climate talks.

The deal falls far short of what was hoped for from Copenhagen and leaves much to be agreed at a follow-up meeting in Mexico to be held sometime next year.

Frustrated leaders turned up to the final day of talks to find no agreed text on the table and spent the last 12 hours trying to reach an agreement.

Leaders of the big emitters - the United States, India and China - are understood to be backing a deal with two major points: a 2050 target of 80 per cent below 1990 levels (which matches what the IPCC says is needed to have a good chance of keeping warming within 2C) and a short-term fund of US$30 billion to be paid by industrialised countries between 2010 and 2012, to help the poorest countries adapt to climate change and curb their emissions.

A spokesman for Prime Minister John Key said the last agreement the Prime Minister saw before leaving the talks included the carrot of US$100 billion a year in aid to be mobilised by rich countries by 2020, however negotiators were still working on the text.

The United States had pledged to create the fund if developing countries - in particular China - signed up to binding emissions cuts that can be checked by international agencies. It is not yet clear where money for the fund will come from.

President Barack Obama this morning declared a "meaningful and unprecedented breakthrough" had been reached among the US, China and three other countries on a global effort to curb climate change but said much work was still be needed to reach a legally binding treaty.

"It is going to be very hard, and it's going to take some time," he said at the conclusion of the 193-nation global warming summit. "We have come a long way, but we have much further to go."

Obama indicated the deal would be accepted by the larger summit in the closing hours.

World short-term targets to 2020 are not included, although developed countries are likely to tack on domestic commitments as an appendix.

New Zealand will have to plead special treatment in order to comply with the global 2050 target. Its target remains cuts of 50 per cent by 2050 - which ministers say is the most New Zealand can do given its heavy reliance on farming and the already large proportion of renewable energy.

John Key left talks at around 10am (10pm in Copenhagen) saying he was comfortable with the draft and that New Zealand would agree to it subject to a few final details.

Mr Key admitted the outcome would be disappointing to developing countries and said there would be those who thought leaders should have done better. "It is progress but I think it falls well short of expectations and aspirations," he said.

Many world leaders had already left.

Delegates, including New Zealand Climate Negotiations Minister Tim Groser, will stay and agree on the final text before countries are invited to sign.

Mr Key said New Zealand could meet the draft agreement without budging from its planned cuts of 10-20 per cent by 2020 and 50 per cent by 2050.

Its contribution to the US$30 billion fund for developing countries would be within the range of NZ$10-50 million a year announced by the Government before the talks.

He said the United States, China, Brazil and India had big roles in drafting the text, which is not legally binding.

It was unclear whether all 193 countries would sign up, and there was doubt over the ability of some countries to meet their pledges, he said.

Rich countries failed to get the two negotiating texts - one for Kyoto parties and one for developing countries - merged into a single document which could have included the United States.

That is a small victory for developing nations, who want Kyoto to carry on but believe that only rich nations should be bound because they carry the lion's share of responsibility for past emissions.

Mr Groser said the process was "extremely frustrating".

Asked whether the next round of talks in Mexico could be brought forward to the middle of next year, as some have suggested, the Prime Minister said it was possible but the next round should not go ahead without broader agreement.

He said too little had been agreed ahead of Copenhagen: "We've turned up to a blank sheet of paper and that makes the starting point very, very difficult" he said.

"If gains are going to be made in Mexico we need to have greater agreement before we get there."

- with AP