Crews may set fires to burn off oil being spewed into the Gulf of Mexico by a blown-out well, the Coast Guard said Tuesday.
Pools of oil far offshore would be trapped in special containment booms and set aflame as soon as Wednesday, Coast Guard Rear Adm. Mary Landry said.
Efforts so far have failed to shut off the flow of oil nearly 5,000 feet below where the rig Deepwater Horizon exploded and sank last week. Robot subs are still working on it.
"If we don't secure this well, this could be one of the most significant oil spills in U.S. history," Landry said.
A similar burn off the coast of Newfoundland in 1993 eliminated 50 to 99 percent of captured oil. Burning the oil could create some air pollution, and the effect on any marine life is unclear.
The oil is now about 20 miles off the coast of Venice, La., the closest it's been to land. But it's still not expected to reach the coast before Friday, if at all.
BP, which was leasing the Deepwater Horizon, said it will begin drilling by Thursday as part of a $100 million effort to take the pressure off the well, which is spewing 42,000 gallons of crude oil a day.
Company spokesman Robert Wine said it will take up to three months to drill a relief well from another rig recently brought to the site where the Deepwater Horizon sank after the blast. Most of the 126 workers on board escaped; 11 are missing and presumed dead. No cause has been determined.
The oil is coming from a pipe rising from the seabed nearly a mile 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 of oil could spill into the Gulf before the relief well is operating. That's 4.2 million gallons. The worst oil spill in U.S. history was when the Exxon Valdez spilled 11 million gallons 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 jump-started efforts Tuesday to contain the spill, which threatens to coat marine mammals and birds with oily slime and taint hundreds of miles of white-sand beaches and rich seafood grounds.
Louisiana-based BP spokesman 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 48 miles long and 80 miles wide at its widest. The borders of the spill were uneven, making it difficult to calculate how many square miles are covered.
Though oil was not expected to reach the coast until late in the week, if at all, concern was growing about what will happen if it does.
In Gulfport, Miss., where white sand beaches are a tourist playground and dolphins, whales and even manatees are frequent visitors to Mississippi Sound, residents were concerned.
Louis Skrmetta, 54, operates the Ship Island Excursions company his grandfather started in 1926. He takes tourists to the barrier islands about 10 miles 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."
Associated Press writers Kevin McGill in New Orleans and Holbrook Mohr in Gulfport, Miss., contributed to this report.