All sides conceded the agreement, which fell far short of United Nations ambitions for the Dec. 7-18 talks, was imperfect but said it was a starting point for a coordinated international effort to avert the catastrophic impacts of climate change.

"This progress did not come easily and we know this progress alone is not enough ... We've come a long way but we have much further to go," Obama said after talks with China's Premier Wen Jiabao, Indian Prime Minister Manmohan Singh and South Africa's President Jacob Zuma which led to the breakthrough.

The agreement still had to win formal approval from a full meeting of all 193 nations at the talks. "If this makes it through the meeting in a couple of hours' time then I see it as a modest success," said Yvo de Boer, head of the U.N. Climate Change Secretariat. "We could have achieved more."

Negotiators had struggled all day to find a compromise acceptable to all which could avert the threat of dangerous climate change, including floods, droughts, rising sea levels and species extinctions.

Tensions between China and the United States, the world's two biggest emitters, had been particularly acute after Obama -- in a message directed at the Chinese -- said any deal to cut emissions would be "empty words on a page" unless it was transparent and accountable.

Obama said that under Friday's agreement, each country would set out "concrete commitments" which would then be subject to "international consultation and analysis".

A U.S. official said it committed nations to carbon emissions cuts to curb global warming to 2 degrees Celsius.

"We're going to have to build on the momentum we've established here in Copenhagen to ensure that international action to significantly reduce emissions is sustained and sufficient over time," Obama said.