The US coastguard is to set fire to oil leaking into the Gulf of Mexico to prevent the slick from reaching shore after last week's explosion on the Deepwater Horizon drilling rig.

Robot submarines have so far failed to shut off the flow more than 1,500 metres below where the Deepwater Horizon was wrecked. Eleven workers are missing, presumed dead, and the cause of the explosion 50 miles off Louisiana has not been determined.

A coastguard spokesman said the burn would begin today. Prentice Danner said fire-resistant booms would be used to corral some of the thickest oil on the surface, which would then be ignited. It was unclear how large an area would be set on fire or how far from shore the fire would be set.

A similar burn off Newfoundland in 1993 eliminated at least half the captured oil.

The National Oceanic and Atmospheric Administration said birds and mammals were more likely to escape a burning area of the ocean than an oil slick. Birds might be disoriented by smoke, but would be at much greater risk from oil in the water.

On the downside, burning the oil creates air pollution and some experts say the effect on marine life is unclear.

Ed Overton, a professor emeritus of environmental sciences at Louisiana State University who is studying the oil spill, questioned whether burning would be successful.

"It can be effective in calm water, not much wind, in a protected area," he said. "When you're out in the middle of the ocean, with wave actions, and currents, pushing you around, it's not easy."

Last night the oil was about 20 miles off Venice, Louisiana, the closest it has been to land, but it is not expected to reach the coast before Friday, if at all.

Hotel owners, fishermen and restaurateurs are keeping anxious watch as the slick spreads towards delicate wetlands, oyster beds and pristine white beaches.

In Washington, the Obama administration launched a full investigation, with authorities saying they would devote every available resource to the inquiry.

The last big spill in the Gulf was in June 1979, when an offshore drilling rig in Mexican waters, the Ixtoc I, blew up, releasing 530million litres of oil. It took until March 1980 to cap the well, and the oil contaminated US waters and the Texas shore.

"In the worst-case scenario, this could also last months," said Richard Haut, a senior research scientist at the Houston Advanced Research Centre.

Thousands of birds such as egrets and brown pelicans are nesting on barrier islands close to the rig's wreckage. If they are affected, rescuers would need to reach their remote islands, wash them down and release them back into the wild.

Michael Ziccardi, director of the Oiled Wildlife Care Network, said cleaning up brown pelican chicks after a modest spill in Louisiana in 2005 was a big undertaking.

"Just about any petroleum can cause problems for birds because they lose their waterproofing, and that's what keeps them dry and warm," he said. "It's a really difficult time, and we're close to the peak of migration."

The spill also threatens billions of fish eggs and larvae.

If the well cannot be closed, almost 100,000 barrels of oil could spill into the Gulf before crews can drill a relief well to alleviate the pressure. The Exxon Valdez tanker, in the worst oil spill in US history, leaked 50 million litres into Alaska's Prince William Sound in 1989.

BP said it would begin drilling a relief well tomorrow even if crews could shut off oil leaking from the underground pipe. A spokesman, Robert Wine, said drilling would take up to three months.

In Pensacola, Florida, the easternmost point likely to be hit by oil, beachgoers and business owners kept watch.

Sal Pinzone, general manager of the fishing pier, arrives at work at 5.30am every day to watch the sun rise over the famous white-sand beach. "We are all worried," he said. "If the spill does hit the beaches along the Gulf, it will shut down everything."