The 194-nation UN-led summit in the Danish capital pledged to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) along with billions of dollars in financing. It gave countries until January 31 to sign on.

China, India, Brazil and South Africa each submitted to the United Nations their plans to fight climate change but described them as voluntary and did not formally endorse the Copenhagen deal.

Todd Stern, the US special envoy on climate change, described the positions of the four developing powers as "a bit ambiguous."

"I do believe that they will sign on to the accord because the consequences of not doing so are so serious -- in a word, leaving the accord stillborn, contrary to the clear ascent their leaders gave to the accord in Copenhagen," Stern said at the Center for American Progress, a Washington think-tank.

The Copenhagen summit in December worked on a framework for fighting climate change after 2012, when developed nations' commitments to curb carbon emissions blamed for global warming run out under the landmark Kyoto Protocol.

Stern refused to predict whether a full-fledged successor to Kyoto would be ready in time for the next major meeting in December in Cancun, Mexico.

"I hope that we can get to a full legal treaty in December, but I'm not going to make any predictions one way or the other," he said. "I'm also not going to fall into the trap of saying if it's not that, we've got a failure."

Developing nations insist that wealthy nations are historically responsible for climate change and therefore should be the only ones with legal obligations.

The United States joined other developed nations in signing on to the Copenhagen accord, a sharp change from the presidency of George W. Bush who rejected Kyoto as unfair.

But President Barack Obama's administration is still trying to overcome opposition in the Senate to approving the first US nationwide plan to cut carbon emissions.

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