The UN Secretary-General is to fly back to Bali tomorrow to help salvage a deal at global climate talks as negotiators worked late into the night to try to break a deadlock between Europe and the United States.

“I will go back to Bali tomorrow morning again to meet with the delegations... and engage myself in continuing further negotiations,” Ban Ki Moon told a press briefing in East Timor, where he was on an overnight visit.

The UN-sponsored talks, originally due to conclude at 6pm (1000 GMT) today, now look set to continue into the weekend, as officials try to resolve a stand-off over the wording of an agreement intended to frame negotiations for a successor to the Kyoto Protocol, which expires in 2012.

The European Union, backed by developing nations, has called for a statement of “ambition” for 25 to 40 per cent cuts in emissions for rich countries by 2020 from 1990 levels. But the United States says such a mention would prejudge the outcome of negotiations.

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Mediators were tonight proposing a compromise which would see countries reaffirm that emissions should be reduced by at least half by 2050.

In an attempt to forestall suggestions of a fudge, Yvo de Boer, the UN climate change chief, told reporters the mid-range 25 to 40 percent was implicit - “an inevitable stop on that road, in the 50-percent goal for mid-century.”

Nevertheless, the EU will no doubt be accused of capitulating to Washington should it accept the proposal, which does not guarantee any level of binding commitment by any nation and marks little progress on the positions adopted at the G8 summit in June in Hamburg.

As the wrangling continued late into the night, however, Yvo de Boer, the UN climate change chief, insisted a deal was close at hand. Negotiators were “on the brink of an agreement,” he told reporters. “Absolutely not deadlocked. People are working very hard to resolve outstanding issues.”

Earlier today both the senior American climate change negotiator and Germany’s Environment Minister said that a compromise was possible. Yesterday, the conference looked certain to end in an acrimonious stalemate, as the EU threatened to boycott a forthcoming climate summit in the United States in protest at Washington’s intransigence. “I’m always optimistic. I think we will have an agreement,” Harlan Watson, President Bush’s top climate negotiator, said on the sidelines of the meeting as talks continued beyond the deadline.

Sigmar Gabriel, the German Environment Minister, agreed: “I think the situation is good and the climate in the climate conference is good and we will have success in the end.”

The 190-nation meeting, which started on December 3, are designed to set a framework for negotiations on succeeding the Kyoto Protocol once the landmark environmental treaty's commitments expire in 2012.

Hillary Benn, the Environment Secretary, emphasised the need for a deal, saying: "If we don't get an agreement here, it's very hard to see how any subsequent process can take that forward."

It was essential that every country in the world was "marching through the door together" if climate change was to be effectively tackled, he said.

The apparent volte face followed a very public attack by Al Gore, the former US Vice-President, on the Bush administration’s position. Speaking at the conference last night, he lambasted the White House for dragging its heels over emissions targets, laying the blame firmly at the feet of President Bush for the apparent failure of the Bali talks.

“I am not an official and I am not bound by diplomatic niceties. So I am going to speak an inconvenient truth: my own country, the United States, is principally responsible for obstructing progress here in Bali,” he said, to the applause of delegates.

Mr Gore, who was awarded the Nobel Prize earlier this year for alerting the world to the dangers of climate change, urged delegates to be patient with the United States, noting that the next occupant of the White House might well take a different attitude to targets.

“Over the next two years, the United States is going to be somewhere it is not now,” he said.

“One year and 40 days from today, there will be a new inauguration in the United States. I must tell you candidly that I cannot promise that the person who is elected will have the position I expect they will have, but I can tell you I believe it is quite likely.”

Kevin Rudd, the recently elected Prime Minister of Australia, overturned his predecessor’s policy and signed up to the Kyoto Protocol within days of entering office, putting pressure on the United States, as the only developed nation to remain outside the pact, to do the same.

“The United States does not want to be seen as the one that is stopping the world from an agreement,” James Leape, head of the WWF conservation group, said.

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