A US official told reporters that a deal had been signed after a multilateral meeting between Obama, Chinese Premier Wen Jiabao, Indian Prime Minister Manmohan Singh and South African President Jacob Zuma. The official acknowledged that the deal is "not sufficient to combat the threat of climate change but it is an important first step".

"No country is entirely satisfied with each element but this is a meaningful and historic step forward and a foundation from which to make further progress," the official added.

Criticism of the deal came fast and furious. Sudanese Ambassador Lumumba Di-Aping, who chairs the G77 group of developing countries plus China, called the agreement "spin" and "insufficient", and indicated the bloc may not agree to the text. He said that the deal announced by the US had "the lowest level of ambition in terms of emissions reductions imaginable" and called it "climate change denial in action."

Di-Aping accused the US and the Danish government of "superimposing a deal on the rest of the world". When asked why important G77 members China, India, Brazil and South Africa had signed up to the text, Di-Aping said that he had yet to speak to representatives from those countries.

He refused to say that the Copenhagen talks had definitively failed and said that talks were continuing, but he said that a deal is more likely to be signed through further negotiations over the next six months.

Environmental groups immediately attacked the accord, with Kumi Naidoo, executive director of Greenpeace International, referring to Copenhagen as the scene of a "climate crime scene".

"Guilty men and women are fleeting to the airport in shame," Naidoo said. "World leaders had a once in a generation chance to change the world for good, to avert catastrophic climate change. In the end they produced a poor deal full of loopholes big enough to fly Air Force One through."

In the meantime, by 1:00 Central European Time large numbers of demonstrators had gathered outside the Bella Centre in Copenhagen where negotiations are still under way.

Obama acknowledged that further climate talks will take many months, and perhaps years, before a legally binding accord is clinched, but he underscored that the US, China, Brazil, India and South Africa have all “agreed to set a mitigation target to limit warming to no more than 2C and, importantly to take action to meet this objective".

Brazilian climate change ambassador Sergio Serra said: “It's very disappointing I would say, but it is not a failure...if we agree to meet again and deal with the issues that are still pending".

EU officials tried to put a brave face on the agreement. A spokesperson says, “a deal is better than no deal”, but other officials said the deal “falls far below our expectations”.

Throughout the evening, the talks centred around disagreements between the US and China over monitoring of carbon emissions, international treaty frameworks, and the level of US emissions pledges. A face saving agreement was only possible after constant mediation efforts by Brazil, and the subsequent meeting of the group made up of China, the US, Brazil, India and South Africa.

Obama was one of the first to leave the conference, leaving EU officials to try and sell the agreement to poorer developing nations, who were thought to be furious with the outcome.