About 10,600 delegates from 186 governments, businesses and environmental groups attended the talks which will last until Dec 12. They discussed a new climate treaty in Copenhagen at the end of 2009 and the Kyoto Protocol.

“It will be an incredible challenge” to reach such a complex accord within a year when the world is struggling with the worst financial crisis since the 1930s, said Yvo de Boer, head of the UN Climate Change Secretariat.

Polish Prime Minister, Donald Tusk, and Denmark's Prime Minister, Anders Fogh Rasmussen, were among speakers at an opening ceremony, along with UN experts.

Mr De Boer praised Mr Obama for saying that he would seek to cut US emissions of greenhouse gases back to 1990 levels by 2020 as part of global action to avert more heatwaves, floods, droughts, more powerful storms and rising seas.

US emissions, mainly from burning fossil fuels in factories, power plants and cars, are about 14 percent above 1990 levels.

President George W. Bush did not ratify Kyoto, saying it would be too costly and excluded targets for developing nations such as China and India. Had Washington ratified, it would have had to cut by seven percent below 1990 levels by 2012.