Barack Obama has thrown his weight behind a deal on climate change by announcing he will attend the Copenhagen summit with a pledge for a 17 per cent cut in US emissions.

The US president's pledge removes one of the final obstacles to a deal, as other developed countries have already announced their targets to cut emissions by 2020. It was less ambitious, however, than similar targets from most other wealthy nations and fell short of environmental groups' hopes.

Mr Obama will attend the early stages of the two-week conference – on December 9 – more than a week before other world leaders arrive on December 17, creating a diplomatic quandary.

“It will be hugely disappointing if he just turns up in the first week and then disappears,” said one official involved in the talks.

“It shows the US is still to one side of the debate – not fully engaged in it,” said Henry Derwent, chief executive of the International Emissions Trading Association and former special envoy on climate change to Tony Blair, Britain's ex-premier. “I don't think [other leaders] will enjoy the prospect of not being able to share the limelight with the US president.”

Greenpeace said it was “the right city, wrong date” and showed that Mr Obama was “just not taking this issue seriously”.

Some suspect Mr Obama of seeking to gain credit for any success at the summit, while being able to distance himself from any failure. But others welcomed the news, saying it boosted the prospects for a deal.

“We really need a target [on emissions] and financial commitment [to provide help for poor countries] – the earlier the better,” said Yvo de Boer, the top United Nations official on climate change. “If he comes in the first week to announce that, it would be a major boost to the conference,”

He said a 17 per cent target on cutting emissions would “help pave the way for a successful outcome at Copenhagen”.

Environmental groups said the target, which represents a cut on 2005 levels, was insufficient. The European Union has pledged to cut emissions by 20 per cent compared with 1990 levels. Using 1990 as a baseline, the US target would represent only a 3 per cent cut.

But Paul Bledsoe, a form- er Clinton White House official now with the National Commission on Energy Policy, said the target – set out in a cap-and-trade bill passed by the House of Representatives – should satisfy other countries.

“The US cannot negotiate beyond the emissions limits in congressional legislation without undermining support for the pending Senate bill,” said Mr Bledsoe.

“By coming the first week, the president is signalling his commitment to helping achieve a global climate deal, while also indicating that expectations for the US position should be realistic, and must be determined by what can be achieved in US law.”

Observers said all eyes were now on China. China and the US are the world's top two emitters, accounting for about 40 per cent of global greenhouse gas output. Beijing is thought to be preparing to make a statement this week on its position regarding cuts.