The US decision was welcomed by many of the delegates at the United Nations Climate Conference in Argentina.

US lobbyist Connie Holmes: Clinton should have waited before signing

However, the move, announced by US Under-Secretary of State Stuart Eizenstat, was criticised by many environmentalists as an empty gesture.

If the Kyoto protocol is to be recognised in American law it must first be ratified by Congress, which has a Republican party majority who have indicated they will not accept the accord in its current form.

Little progress

Correspondents said there was little sign of agreement as the Buenos Aires climate conference entered its final day on Friday.

The 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.

Listen to Stuart Eizenstat on BBC World TV

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.