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VENICE, La.—Coast Guard crews in the Gulf of Mexico Wednesday set fire to a portion of an oil slick spreading from a deepwater well, which the agency said was leaking at a far greater rate than it had believed.

Rear Adm. Mary Landry said that 5,000 barrels a day were now estimated to be leaking, five times the previous estimate of 1,000 barrels, after the agency found a significant new oil leak.

Ms. Landry said she briefed President Barack Obama and that the government has offered the services of the Department of Defense to help contain the spill.

The upgraded estimates, announced late Wednesday night, came after the National Oceanic Atmospheric Administration studied aerial surveys of the oil's trajectory, Ms. Landry said.

Doug Suttles, chief operating officer for BP Exploration and Production, which leased the rig, said the location of the newly detected leak was just atop an automatic shutoff mechanism —known as a blowout preventer—that the company has been trying to activate since Sunday to shut off the well.

The spill could strike the Louisiana coast before the end of the week, Mr. Suttles added.

Mr. Suttles didn't confirm the Coast Guard's 5,000-barrel estimate. "We do not believe this changes the amount currently estimated to be released,'' Mr. Suttles said.

The burning technique conducted earlier in the day was deemed by experts to be the safest, cheapest way to limit environmental damage. But it has never been tried on this scale.

"Burning oil on the water is the most environmentally friendly technique that one can think of," said combustion expert Anil Kulkarni at Pennsylvania State University, who has studied the technique for the National Institutes of Standards and Technology.

The controlled oil fire at sea started Wednesday evening in a small area of the gulf spill and lasted for a little over an hour, said Coast Guard spokesman David Mosley.

Results of the test burn weren't immediately disclosed. These oil fires pose some problems. Smoke plumes from the burning oil are thick with soot. Typically, there are also traces of toxic sulfur dioxide, nitrogen dioxide and carbon monoxide.

However these tend to dissipate quickly, say researchers who have studied the process, and the toxic oil compounds that might be most harmful to wildlife are consumed in the fire.

Experiments conducted by the Canadian government have shown that the chemical byproducts of oil burning appear to have little effect on marine species.

Although the spill already covers an area about the size of Jamaica, crews can burn only a small portion at a time.

They have to corral a patch of oil with fireproof booms and then ignite it with a flare—a job made harder by wind and waves. To keep the burning oil contained, they then must tow the entire setup, so that the current keeps the burning oil floating between the booms.

"We are trying to light on fire wet oil in the middle of the ocean," said Chris Reddy, who studies oil spills as director of the Coastal Ocean Institute at the Woods Hole Oceanographic Institution in Massachusetts.

It already may be too late to incinerate the oldest portions of the spill, which has been growing from damaged well pipes 5,000 feet underwater since the Transocean rig Deepwater Horizon caught fire and sank last week.

Researchers have experimented with the burning technique since it was first tried after the spill from the tanker Exxon Valdez in 1989.

Crews are also spreading thousands of gallons of chemical dispersants—usually detergents or alcohol-based solvents—to speed the rate at which the oil dissolves in seawater.

In Venice, a fishing and oil-field-services outpost at the southern tip of Louisiana that is the closest community to the oil spill, residents and local officials hoped that the controlled burning would help keep the crude away from the fragile ecosystem and valuable fisheries.

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VENICE, La.—Engineers prepared to try containing the gushing Gulf of Mexico oil well with giant underwater boxes and siphons, as seaside towns braced for landfall of a giant slick.

BP PLC, the oil giant that leased the rig whose sinking last week caused the disaster, has failed in efforts using unmanned submarines to activate a shutoff device on the undersea well.

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A stopgap solution BP is planning—covering the well with containers and pumping the oil out—will take weeks to roll out and is untested at the one-mile depth of this well, however. BP said it would begin working this weekend on a permanent solution to the crisis, drilling a new hole to cut off the damaged well, but industry scientists said that could take months.

The Deepwater Horizon, operated for BP by Transocean Ltd., burned and sank last week, leaving 11 dead and an open well on the ocean floor.

With a quick solution to shut off the spill looking out of reach Friday, the government and the oil industry struggled to contain the resulting slick and keep it from shore. The American Petroleum Institute alerted members that Interior Secretary Ken Salazar wanted advice from the industry on how to manage the spill by the end of Friday.

On Friday evening, the National Guard was mobilized to assist in the cleanup, and the Pentagon said BP would have to bear the cost. Earlier Friday, a small drilling rig tipped over in inland waters near Morgan City, La., the Coast Guard said, though no oil was spilled.

The Deep Horizon slick began threatening the wetlands of the Louisiana coast, raising fears of environmental disaster in some of America's richest shrimp, oyster and fish breeding grounds.

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Strong winds and choppy seas hampered efforts to hem in the oil. Several vinyl containment barriers, known as booms, broke up in the rough weather. Others remained on shore, as high waves—expected to continue through the weekend—made it impossible to lay them in the Gulf.

An equally pressing emergency loomed more than 40 miles offshore, where the deepwater well kept spewing oil uncontrollably.

Industry scientists say the permanent solution is to close the entire well. To do that, they must drill another hole—through 13,000 feet of rock a mile under the ocean's floor—that will intercept the leaking well. They can then pump in cement to try to plug the leaks.

Timeline

This operation will take up to three months and is highly complex; the drills must precisely hit the leaking well—which is just seven inches wide. When a well off the coast of Australia blew out last year, it took five attempts over 10 weeks to hit the old well and shut it down.

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BP says it will begin drilling the new well this weekend.

Within hours of the explosion, BP was sending unmanned submarines to the well to try to trigger a device called a "blowout preventer," which is essentially a powerful valve meant to clamp down on the well and shut it off in case of emergency.

The device should have been triggered in the explosion, but wasn't; that failure will be a central question in the investigation.

In theory, the blowout preventer can also be activated by underwater robots. BP has six robots working on it, and says it will keep trying, but so far the valve has not worked. "It's just not functioning appropriately," Tony Hayward, CEO of BP, said in an interview this week.

For now, BP is trying a series of stopgap measures. The company is constructing three steel boxes—each 40 feet tall and weighing 73 tons—that it will place on top of the gushing oil. Pipes running through the boxes will carry the oil to a ship.