Officials are considering setting fire to an oil spill in the Gulf of Mexico as efforts to stem a leak from a rig that exploded and sank are failing.
The coast guard is concerned that, unless controlled, the leak could cause one of the worst spills in US history.
The "controlled burn" could start later far from shore, said Coast Guard Rear Adm Mary Landry, who is in charge of the US clean-up effort.
She said work on sealing leaks using robotic submersibles could take months.
About 1,000 barrels (42,000 US gallons; 35,000 imperial gallons) of oil a day have been gushing into the sea since an explosion on the Deepwater Horizon platform last week caused the rig to sink.
Eleven of the rig's workers are still missing and presumed dead in the disaster off the Louisiana coast.
Adm Landry warned that if the well was not secured soon, "this could be one of the most significant oil spills in US history".
A "controlled burn" would involve setting fire to an area of petroleum trapped by special containment booms on the water's surface.
Environmental experts say birds and animals are more likely to escape a burning patch of water than an oil slick, although toxic fumes could endanger wildlife.
"We fully understand there are benefits and tradeoffs," said Adm Landry.
But she noted that with the spill moving toward land, the impact on Louisiana's coastline, which contains some 40% of the nation's wetlands and spawning grounds for countless fish and birds, had to be considered.
Controlled burns had been tried and tested before, and had been shown to be "effective in burning 50 to 95% of oil collected in a fire boom", she said.
The leaks - about 5,000ft (1,525m) under the surface - were found on Saturday, four days after the Deepwater Horizon platform, to which the pipe was attached, exploded and sank.
The resulting oil slick now has a circumference of about 600 miles (970km) and covers about 28,600 sq miles (74,100 sq km).
The slick is now about 20 miles (32km) off the coast of Louisiana, but wind projections indicate it will not reach land before Saturday.
It would have to continue for more than eight months to match the 11m-gallon spill from the oil tanker Exxon Valdez off Alaska in 1989.
Workers on a nearby oil platform were evacuated by the US authorities on Monday after the oil slick came dangerously close.
British oil company BP, one of the firms operating the rig, has not been able to activate a device known as a blow-out preventer, designed to stop oil flow in an emergency.
Doug Suttles, the chief operating officer for exploration and production at BP, said it had not yet given up on engaging the valve, but was considering other possible solutions.
These include placing a dome directly over the leaks to catch the oil and send it up to the surface, where it could be collected by ships. This has only been done in shallow water before and is still two to four weeks from being operational.
BP will also begin drilling a "relief well" intersecting the original well, but it is also experimental and could take two to three months to stop the flow.
Forty-nine vessels - oil skimmers, tugboats barges and special recovery boats that separate oil from water - were working to round up oil, BP said.
An investigation has been ordered into the cause of the leak by the interior and homeland security departments.
It will have the power to compel witnesses to testify, and will look into possible violations by the operators of the rig, Transocean. - BBC