World leaders are edging towards a climate change deal in Copenhagen - but a new draft agreement has allegedly dropped a deadline of 2010 for reaching a legally binding treaty.
US President Barack Obama says the chance to agree a global deal "hangs in the balance" and that an "imperfect deal" was better than nothing.
The key issues dividing rich and developing countries include who should cut their emissions and by how much, the scale of finance to help the poorest nations fight climate change and monitoring of each country's greenhouse gas reductions.
Today is the final day of the two-week summit, which has been mired in wranglings and walk-outs, meaning negotiators were unable to produce an agreed text for the arriving leaders to discuss.
They were locked in talks until 2am today but by the time the conference reconvened, last night’s optimism about a possible deal evaporated as it became clear there was still no sign of progress.
Mr Obama warned today that the chance of a global climate change deal “hangs in the balance” as he joined other world leaders at deadlocked talks in Copenhagen.
But the US president offered no new commitments on emissions targets which many hoped for to end the impasse after two weeks of tortuous negotiations.
In a speech to the conference, before leaders were due to reconvene a tense meeting aimed at producing a political agreement, he said their capacity to do so appeared to be in doubt.
In an appeal to fellow leaders to find a solution, he said: “The question before us is no longer the nature of the challenge; the question is our capacity to meet it.
“For while the reality of climate change is not in doubt, I have to be honest, as the world watches us today, I think our ability to take collective action is in doubt right now and it hangs in the balance.”
Mr Obama insisted that the actions the US was taking, including cuts of 17% on current levels by 2020, were ``ambitious'' and were being taken in the interests of America's economy and security, as well as to tackle climate change.
Laying out what he believed should be in a deal, the president said all major economies needed to take national action to reduce emissions, and there must be mechanisms to review and exchange information on the steps taken – which need not be “intrusive or infringe on sovereignty”.
But he said that without accountability on actions to cut emissions, any agreement would be “empty words on a page”.
He said: “I don’t know how you have an international agreement where we are all not sharing information and ensuring we are meeting our commitments. That doesn’t make sense. It would be a hollow victory.”
He reiterated America’s commitment to a 10 billion US dollar-a-year “fast start” package and long-term funding to the tune of 100 billion US dollars a year by 2020.
And he urged leaders to work together rather than fall back into discussions and debate that had gone on for decades.
“We know the fault lines because we’ve been imprisoned by them for years. But here is the bottom line: we can embrace this accord, take a substantial step forward, and continue to refine it and build upon its foundation.
“We can do that, and everyone who is in this room will be a part of an historic endeavour – one that makes life better for our children and grandchildren.
“Or we can again choose delay, falling back into the same divisions that have stood in the way of action for years.
“And we will be back having the same stale arguments month after month, year after year – all while the danger of climate change grows until it is irreversible,” he warned.
Addressing the high-level conference, Chinese premier Wen Jiabao insisted his country took the issue of climate change very seriously and has made, and would continue to make, “unremitting efforts to tackle this challenge”.
He said that its target to curb greenhouse gas growth – to reduce the amount of emissions per unit of GDP by 40% to 45% by 2020 – came without any conditions attached and the Chinese would honour it.
And he said: “We remain fully committed to achieving and even exceeding the target.”
He added China would increase the transparency of the actions it was taking to curb its emissions growth.
Mr Obama, who flew in to join the summit this morning, had been tipped by senior figures to arrive with further commitments from the US, perhaps a rise in its harmful emissions reduction target from 17% on present levels.
Shortly before his speech, he joined the end of a two-and-a-half-hour session of talks which broke up while fresh efforts were made to draft an agreement acceptable to all sides.
Around 30 of the key players locked themselves into a room without advisers for a fresh effort to find a way forward as the scheduled end of the conference loomed.