BP, which leases the Deepwater Horizon platform, has been operating four robotic submarines some 1,500 meters (5,000 feet) down on the seabed to try to cap two leaks in the riser pipe that connected the rig to the wellhead.

But the best efforts of the British energy giant have yielded no progress so far, and engineers are frantically constructing a giant dome that could be placed over the leaks as a back-up plan to try and stop the oil spreading.

Time is running out as a huge slick with a 600-mile (965-kilometer) circumference has moved within 21 miles of the ecologically fragile Louisiana coast despite favorable winds.

The US authorities said they were considering a controlled burn of oil captured in inflatable containment booms floating in the gulf to protect the shorelines of Louisiana and other southern states.

"I am going to say right up front: the BP efforts to secure the blowout preventer have not yet been successful," Rear Admiral Mary Landry told a press conference, referring to a 450-tonne machine that could seal the well.

Asked to compare the accident to the notorious 1989 Exxon Valdez oil tanker disaster, Landry declined but said: "If we don't secure the well, yes, this will be one of the most significant oil spills in US history."

The US government promised a "comprehensive and through investigation" into the deadly explosion that sank the platform and pledged "every resource" to help stave off an environmental disaster.

The rig, which BP leases from Houston-based contractor Transocean, went down last Thursday 130 miles southeast of New Orleans, still burning off crude two days after a blast that killed 11 workers.

The widow of one of the dead crew members has filed a lawsuit accusing the companies that operated the rig -- BP, Transocean and US oil services behemoth Halliburton -- of negligence.

The blast created a slick that could reach Louisiana's wetlands -- which are a paradise for rare waterfowl and other wildlife -- within days if the winds change.

"It is the closest it's been to shore through this response," said Landry.

BP has sent a flotilla of 49 skimmers, tugs, barges, and recovery boats to mop up the spill, but their efforts were hampered at the weekend by strong winds and high seas.

A rig is on stand-by to start drilling two relief wells that could diverting the oil flow to new pipes and storage vessels.

But BP officials say the relief wells will take up to three months to drill, and with oil gushing into the Gulf of Mexico at the rate of 42,000 gallons a day, the dome is seen as a better interim bet.

US coast guard spokesman Prentice Danner told AFP the dome will take two to four weeks to build.

"This is the first time this has ever been done. This idea didn't exist until now. It has never been fabricated before," he said.

The exact dimensions and design of the dome were still being worked out, but officials said it would be similar to welded steel containment structures called cofferdams that are already used in oil rig construction.

"If you could picture a half dome on top of the leak and the oil collects inside of this dome and is pumped out from there, that is the idea behind it," explained Danner.

BP chief executive Tony Hayward expressed confidence an environmental disaster would be averted as he acknowledged that strong first-quarter results Tuesday had been overshadowed by the "tragic accident."

According to National Oceanic and Atmospheric Administration (NOAA) experts participating in the spill response, the slick is "very thin" and consists of "97 percent sheen."

Northwest winds blowing the oil away from Louisiana were predicted to keep the slick from reaching shore through Thursday at least.

Communities in southern US states along the Gulf of Mexico were nonetheless bracing for the possibility of polluted beaches and fisheries that are crucial to the region's economy.

Landry noted that the deadly rig accident has not disrupted offshore gulf oil production -- which accounts for one-third of the US energy supply.

Louisiana Governor Bobby Jindal, meanwhile, ordered all flags over state buildings to fly at half-staff through sunset May 3 as a token of respect for the 11 crew members who remain missing and are presumed dead.

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