Key states have reached what they call a "meaningful agreement" at the Copenhagen climate summit.
A US government official said the deal was a "historic step forward" but was not enough to prevent dangerous climate change in the future.
Analysts welcomed the fact that a deal had been done, but said its achievements were modest.
US President Barack Obama said the deal would be a foundation for global action but there was "much further to go".
He said the US, China, Brazil, India and South Africa had "agreed to set a mitigation target to limit warming to no more than 2C and, importantly, to take action to meet this objective".
He added: "We are confident that we are moving in the direction of a significant accord."
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