US president Barack Obama has warned 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 including Prime Minister Gordon Brown 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."