The report by the General Accounting Office, Congress' investigative arm, adds another wrinkle to a debate over a Republican-backed plan to allow drilling in the nearby Arctic National Wildlife Refuge (ANWR), which Democrats have vowed to defeat on environmental grounds.

The GAO prepared the report at the request of House Minority Leader Richard Gephardt and Massachusetts Rep. Edward Markey, two Democrats taking part in House-Senate negotiations to reach a compromise on wide-reaching energy legislation.

Since the opening of the Trans-Alaska Pipeline in 1977, more than 13 billion barrels of oil have flowed from the region, supplying about 20 percent of U.S. annual production.

However, once the wells end production, oil companies will face a big cleanup bill, totaling between $2.7 billion and $6 billion, the GAO said.

As of January, only BP and Phillips operated fields there, while Exxon Mobil Corp, Anadarko Petroleum Corp and ChevronTexaco Corp hold interests in some wells there.

In a letter included in the report, the head of Alaska's Department of Natural Resources said it was not "self-evident that it is better to adopt specific standards today for ... activities that may not take place for half a century."

British Petroleum, Phillips Petroleum Co. and others have collectively spent $53 billion to build gravel roads, pipelines and airstrips for oil production from the Utah-sized region known as the North Slope.

Oil firms say they have deep enough pockets to fund an eventual North Slope cleanup.

"BP has the resources and has made a commitment to handle (cleanup) on the North Slope," a company spokesman said.

BP does not reveal its specific North Slope cleanup liabilities. In 2001, BP estimated its total global cleanup-related liabilities at $5.3 billion over the next 30 years, the spokesman said.

The land was leased by the oil companies from the Interior Department and the state of Alaska. Under the law, public lands must be restored after drilling ceases.

The Interior Department and oil companies "have a very lax definition of what will be required and how much they will have to pay," Markey said, referring to cleanup costs.

The GAO said the cleanup will be an "enormous undertaking," especially in the area's hostile Arctic climate - the sun never breaches the horizon for two months during the winter.

The state of Alaska has advanced no specific dismantlement plans for North Slope wells. Until then, GAO said it is not possible to give an accurate estimate of how much it will cost to dismantle the drilling sites.

Funds provided by the oil industry in shape of bond requirements will yield only a "small portion" of potential funds needed and are "insufficient" to ensure proper restoration, the GAO said. Oil firms must put up $200,000 for each Alaska drilling operation and $500,000 for all of its oil and gas leases in the state, it said.

To get bond backing to cover cleanup costs, energy firms would have to go to a bond company "that would likely have less financial wherewithal than the oil companies themselves," said Bob King, a spokesman for Alaska Governor Tony Knowles.