Crews are to begin drilling in the Gulf of Mexico by Thursday to reduce the pressure from a blown-out well off the Louisiana coast that is spewing 159,000 litres of crude oil a day into the Gulf.

This aerial photo taken Monday near the coast of Louisiana shows some of the oil that's leaked from a pipeline since last week's explosion and collapse of the Deepwater Horizon oil rig. (Gerald Herbert/Associated Press)

The Deepwater Horizon, the offshore oil rig that exploded last week, triggering the spill, was leased by British oil giant BP. Company spokesman Robert Wine said Tuesday it will cost $100 million US and take up to three months to drill a relief well from another rig brought to the site where the Deepwater Horizon sank after the blast.

Most of the 126 workers on board escaped, but 11 are missing and presumed dead. No cause has been determined.

The Obama administration said it has launched a full investigation into the explosion. Homeland Security Secretary Janet Napolitano and Interior Secretary Ken Salazar said they will devote every available resource to a comprehensive investigation.

Unable to stop flow

The oil is coming from a pipe rising from the seabed about 1.6 kilometres underwater. So far, crews using robotic subs have been unable to activate a shutoff device at the head of the well. A kink in the pipe is keeping oil from flowing even more heavily.

If the well cannot be closed, almost 100,000 barrels, or 15.9 million litres, of oil could spill before the relief well gets up and running. The worst oil spill in U.S. history occurred when the Exxon Valdez spilled 41.6 million litres in Alaska's Prince William Sound in 1989.

BP said it will drill the relief well even if it is able to shut off the flow of oil.

Improving weather on Tuesday jump-started efforts to contain the spill, which threatens to coat marine mammals and birds with oily slime and taint hundreds of kilometres of white-sand beaches and rich seafood grounds.

Louisiana-based BP spokesperson Neil Chapman said 49 vessels — oil skimmers, tugboats, barges and special recovery boats that separate oil from water — are working to round up oil as the spill area continues to expand.

As of Tuesday morning, oil that leaked from the rig site was spread over an area about 80 kilometres long and 130 kilometres wide at its widest. The borders of the spill were uneven, making it difficult to calculate how many square kilometres are covered.

Coastal concerns

Though oil is not expected to reach the coast until late in the week, if at all, concern has been growing about what will happen if it does.

In Gulfport, Miss., where white sand beaches are a tourist playground and dolphins, whales and manatees are frequent visitors to Mississippi Sound, residents braced for the worst.

Louis Skremette, 54, operates the Ship Island Excursions company his grandfather started in 1926. He takes tourists to the barrier islands about 16 kilometres south of Gulfport in the Gulf Islands National Seashore. Its powder-white beaches and clear, green water create an idyllic setting for sunning and observing marine birds and sea life.

He sees the advancing spill as a threat to everything important in his life.

"This is the worst possible thing that could happen to the Mississippi Gulf Coast," he said. "It will wipe out the oyster industry. Shrimping wouldn't recover for years. It would kill family tourism — that's our livelihood."