Washington - Top climate officials declared themselves slightly more hopeful of reaching a global deal on combatting global warming by the end of this year, after a meeting Tuesday of the world's 17 leading economies.

But the two-day Washington conference made little progress in resolving the major disagreements - between the United States, Europe and emerging powers including China and India - over what that global agreement should look like.

US climate-change envoy Todd Stern played down the differences and described the Major Economies Forum on Energy and Climate Change as a "trust-building" exercise. It was the first meeting of the group since President Barack Obama took office in January.

"I come out of this meeting if anything a bit more optimistic" about reaching a deal by a major UN summit in December in Copenhagen. But Stern added: "I don't ever underplay the size of the challenges here."

Other countries said they welcomed the new atmosphere under an Obama administration that has set about reversing many positions of former president George W Bush, who opposed aggressive targets on cutting US greenhouse-gas emissions during his eight years in office.

"The US is fully back in the debate, and because of that we are back in business," said Joao Vale de Almeida, European Commission envoy to the talks.

The European Union had felt "a bit lonely" in the past in pressing for more action.

German Environment Minister Sigmar Gabriel said the EU and United States remained "far apart," and Stern acknowledged there was little movement on the main issue: How much countries should lower their climate-damaging emissions.

Obama has pledged to lower the greenhouse-gas emissions blamed for global warming by about 15 per cent by 2020 compared to 2005 levels and 80 per cent by 2050. The administration is facing stiff resistance in getting legislation through Congress to embrace those ambitious targets.

The European Union is aiming for much sharper cuts with a plan that would reduce emissions by 20 per cent by 2020 compared to 1990 levels. The US would have to lower its emissions by about 30 per cent over the same period to comply.

Developing countries are resisting hard targets on their own emissions, insisting that wealthier nations bear the most historical responsibility and should bear the brunt of the effort to combat climate change.

Almeida said there would be "no real solution" to climate change without a "substantial contribution" from emerging powers. China has become the world's largest polluter together with the United States, though its per-capita emissions remain far below that of wealthier countries.

Officials from the 17 top polluters, which together make up more than 80 per cent of the world's emissions, have said they want to get the outlines of an agreement within their group in time for a summit of leaders in July in Italy. The world's governments hope to get a final deal in place by the Copenhagen summit in December.

The 17 environment ministers will meet again in May and June before the Italy summit. Officials on Tuesday said it was likely the talks would continue well into the second half of 2008.

Some climate groups described the Washington conference as a lost opportunity to make headway.

"Unfortunately, while the last two days brought soaring and inspiring rhetoric reflecting the profound sea change in US climate policy ­ it brought little in the way of real progress," said Carroll Muffett of Greenpeace.