Proposals by China and the US, the world's two biggest carbon emitters, for tackling greenhouse gases boosted prospects for a deal at next month's global- warming summit, the UN's said climate chief on Thursday.

"The US commitment to specific, mid-term emission cut targets and China's commitment to specific action on energy efficiency can unlock two of the last doors to a comprehensive agreement," Yvo de Boer said in a statement.

De Boer is executive secretary of the UN Framework Convention on Climate Change (UNFCCC), organising the 7-18 December talks in Copenhagen, which are aimed at forging a new global pact on tackling the greenhouse-gas crisis.

He warned, though, that more needed to be done, especially from developed economies.

"Let there be no doubt that we need continued strong ambition and leadership. In particular, we still await clarity from industrialised nations on the provision of large-scale finance to developing countries for immediate and long-term climate action."

On Thursday, China ? the world's number one emitter and most populous country ? announced it would cut the amount of carbon dioxide (CO2) emitted per unit of gross domestic product in 2020 by between 40 and 45 percent, based on 2005 levels.

A day earlier, the White House said President Barack Obama would attend the talks on 9 December putting forward an offer to cut US emissions by 17 percent from 2005 levels by 2020.

The goal of the Copenhagen meeting is an outline accord for reducing carbon emissions beyond 2012 and channelling funds to poor countries so that they can avoid treading a high-carbon path and shore up their defences against climate change.