Washington - The US House of Representatives could vote late Friday on a historic climate bill that for the first time would force companies to limit greenhouse-gas emissions blamed for global warming.

The prospects for the legislation are unclear: Congress is deeply divided and a flurry of lobbying by supporters and opponents was underway throughout this week.

President Barack Obama, who has made global warming legislation a top priority of his administration, was on the telephone Thursday with key legislators still on the fence.

Climate groups, business groups, industry groups and others have all been out in force this week with television advertisements and mobilizing their supporters to call their congressional representatives.

The legislation would create a so-called cap-and-trade system, which already exists in Europe and would create a market for carbon dioxide emissions that cause global warming. The idea is to give companies pollution allowances that can be traded between dirtier and cleaner firms.

The hope is a cap-and trade system will cut US emissions 17 per cent below 2005 levels by 2020 and nearly 80 per cent by 2050. But critics say it will place an undue burden on the US economy.

The United States, which together with China emits about half of the world's greenhouse gases, has faced pressure from other governments to more aggressively reduce its carbon footprint.

What the United States does is also considered key to reaching an agreement an the Copenhagen summit in December, where the world's governments hope to agree on a new global treaty to combat climate change.(dpa)