US Under-Secretary of State Stuart Eizenstat announced the decision at the Buenos Aires conference.

He said: "In taking this action, the United States reaffirms our commitment to work with the nations gathered here to address the challenge of climate change," he said.

Listen to Stuart Eizenstat on BBC World TV

The announcement drew a smattering of applause from the conference floor.

US congressional leaders have indicated they won't accept the Kyoto accord in its current form.

US lobbyist Connie Holmes: Clinton should have waited before signing

Critics in the Republican-dominated Congress fear cuts in emissions will affect the US economy.

}They have warned the White House against signing the pact, saying it could galvanise opposition.

A wealth of differences

The Buenos Aires conference began with hopes that ground rules would emerge on how to limit carbon dioxide pollution, which is blamed for global warming.

Tim Hirsch in Buenos Aires: "America has stressed that the signing imposes no obligations"

The main dispute is between the United States and Europe over emissions trading - the right of countries which fulfil pollution cutting commitments to sell their excess allowances to nations struggling to meet their objectives.

European countries favour a tax to make it harder for rich countries to buy permits abroad to avoid making cuts at home. The US argues emissions trading should operate under free-market principles.

Many countries also fear they losing out to competitors if they adopt potentially expensive pollution controls and others do not.

At the Kyoto meeting last year, developed nations made a pledge to cut their emission of greenhouse gases to 5.2% below their 1990 level by 2008-2012.

In Argentina, differences have also been exposed between industrialised and developing nations over the best way forward.

Large developed nations such as the US want voluntary commitments from developing countries to cut their greenhouse gases.