With the clock ticking on the high-stakes Copenhagen climate summit, US President Barack Obama will try to salvage fading hopes for a deal as he meets this month with the leaders of China and India.

Obama on Sunday starts his closely watched debut trip to China. A week later, Indian Prime Minister Manmohan Singh comes to Washington in the first full-fledged state visit of Obama's presidency.

The United States is also dispatching Energy Secretary Steven Chu to both emerging powers in hopes of making headway ahead of the December 7-18 summit in the Danish capital.

The world's three most populous nations have all vowed action on climate change but are deeply at odds over the shape of a Copenhagen deal, which was meant to be a new global treaty but now looks set to offer a framework at best.

German Chancellor Angela Merkel has even threatened to boycott Copenhagen unless the three nations move forward on their positions.

"A failure of the world climate conference in Copenhagen in December would set international environmental efforts back by years. We cannot afford this," Merkel said.

Obama has sharply changed US climate policy but, like his predecessor George W Bush, has joined the Europeans, Japan and other rich nations in demanding that China and India act.

"No country holds the fate of the Earth in its hands more than China," Todd Stern, the US climate envoy, told a recent congressional hearing.