WASHINGTON/COPENHAGEN - The United States took a step on Monday towards curbing its greenhouse gas emissions, boosting the start of negotiations between 190 countries in Copenhagen on a deal to combat global warming.

The U.S. Environmental Protection Agency ruled that greenhouse gases endanger human health, allowing it to regulate them without legislation from the Senate, where a bill to cut U.S. emissions by 2020 is stalled.

The ruling was welcomed at the opening day of Dec. 7-18 talks in Copenhagen, where a record 15,000 participants are trying to work out the first new U.N. pact in 12 years to combat rising seas, desertification, floods and cyclones.

"This is very significant in the sense that if...the Senate fails to adopt legislation (on emissions), then the administration will have the authority to regulate," Yvo de Boer, head of the U.N. Climate Change Secretariat, told Reuters.

A White House spokesman said President Barack Obama still preferred legislation, rather than regulation, to curb planet-warming gases.

The United States is the number two emitter behind China and is alone among industrialised nations outside the existing Kyoto Protocol that curbs emissions until 2012. Kyoto was meant as a small step to avert the feared consequences of climate change, which also include heatwaves and droughts.

DEAL "WITHIN REACH"

The Copenhagen talks opened with a stark U.N. warning about the risks and a prediction by Denmark's Prime Minister Lars Lokke Rasmussen that a deal to combat climate change is "within our reach".

He said that 110 world leaders including Obama would attend a closing summit. The planned presence of so many leaders meant "an opportunity the world cannot afford to miss," he said.

Many nations say that the United States is the key to a deal in Copenhagen to break deadlock between rich and poor nations about sharing out the burden of curbs on greenhouse gas emissions. Most emissions are from burning fossil fuels.

Obama is aiming to cut U.S. emissions by 3 percent below 1990 levels by 2020, with far deeper cuts in coming decades.

Many developing nations want far more from Washington by 2020.

"President Obama and the United States Congress can now travel to Copenhagen armed with regulatory credibility and emission reduction targets," said Edward Markey, a co-author of a carbon capping bill that passed the House of Representatives.

"The world is watching, and the United States is acting."

Politicians and scientists urged Copenhagen delegates to agree immediate action to curb emissions and come up with billions of dollars in aid and technology to help poorer countries limit their emissions.

Rajendra Pachauri, the head of the U.N.'s panel of climate scientists, said action was needed to avoid cyclones, heatwaves, floods and possible loss of the Greenland ice sheet, which could mean a sea level rise of 7 metres (23 feet) over centuries.

"The evidence is now overwhelming that the world would benefit greatly from early action," he said.

He defended the findings by his panel after leaked emails from a British university last month led sceptics to say that researchers had conspired to exaggerate the evidence. He said there were rigorous checks on all research.

The European Union said it may sharpen its carbon-cutting proposal if the United States paid for more carbon cuts in poor nations, especially to curb deforestation.

Developing nations including small island states, which are most vulnerable to rising sea levels, demanded more action.

"So far we have not seen any real leadership" from rich nations, said Ibrahim Mirghani Ibrahim of Sudan, speaking on behalf of the Group of 77 and China.

Dessima Williams, of Grenada, speaking on behalf of small island states at risk from rising seas, said the group "will not accept a made-for-television solution...We are here to save ourselves from burning and from drowning."

China's climate ambassador Yu Qingtai told Reuters: "the parties are still pretty much wide apart, especially on some of the critical substantive issues."

China, Brazil, South Africa and India - the four major emerging economies responsible for 30 percent of global carbon emissions - want a global climate treaty wrapped up by June 2010, according to a joint draft document, prepared for the Copenhagen talks and obtained by Reuters.

Some other countries have suggested an end-2010 deadline.

Outside the conference centre, delegates walked past a slowly melting ice sculpture of a mermaid, modelled on the Danish fairy tale of "The Little Mermaid", as a call for action.