The US Coast Guard and BP gave an update on the crisis
The US military has joined efforts to stop an oil leak in the Gulf of Mexico as fears rise about its scale.
Five times as much oil as previously thought could be leaking from the well beneath where a rig exploded and sank last week, the US Coast Guard says.
Rear Admiral Mary Landry said some 5,000 barrels (210,000 gallons) a day were thought to be gushing into the sea 50 miles (80km) off Louisiana’s coast.
The slick is 45 miles by 105 miles and heading towards the coast.
If the coastguard estimate is correct, the spill could match the 11m gallons spilt from the Exxon Valdez tanker off Alaska in 1989 within two months.
Oil giant BP operated the Deepwater Horizon rig. Its chief operating officer of exploration and production, Doug Suttles, welcomed the US military’s offer of help.
He said the company was using remote operative vehicles (ROVs) to try to find out how much oil was leaking into the sea.
“This is very, very difficult to estimate,” Mr Suttles told reporters.
“Down below the surface we actually can’t meter this oil so we can just observe it… what our ROV pictures show to us on the sea floor hasn’t changed since we first saw the leak… but what we can say based on what we’re picking up on the surface it looks like it is more.”
Mr Suttles estimated something between 1,000 and 5,000 barrels a day was leaking.
Meanwhile, a fire-fighting expert said the disaster may become the biggest oil spill ever.
Mike Miller, head of Canadian oil well fire-fighting company Safety Boss, told the BBC World Service: “Probably the only thing comparable to this is the Kuwait fires [following the Gulf War in 1991].
“The Exxon Valdez is going to pale in comparison to this as it goes on.”
Scientists say only a quarter of local marine wildlife survived the Exxon Valdez disaster.
The scale of the operation to contain the oil spill and protect both the US coastline and wildlife is unprecedented, with the military and other government agencies collaborating with BP – which had hired the sunken rig – and industry leaders.
Efforts to stem the flow are being complicated by the depth of the leak at the underwater well, which is about 5,000ft (1,525m) beneath the surface.
Weather forecasters have meanwhile warned that changing winds could drive the oil slick ashore by Friday night. Its leading edge is now only 20 miles (32km) east of the mouth of the Mississippi.
A coastguard crew has set fire to part of the oil slick in an attempt to save environmentally-fragile wetlands.
The “controlled burn” of surface oil took place in an area about 30 miles (50km) east of the Mississippi River delta.
But Mr Miller warned that burning off leaking oil was not a long-term solution.
“The object of this game is to shut off the flow,” he said.
Engineers are working on a dome-like device to cover oil rising to the surface and pump it to container vessels, but it may be weeks before this is in place.
It is feared that work on sealing the leaking well using robotic submersibles might take months.
BP is also working on a “relief well” to intersect the original well, but this is experimental and could take two to three months to stop the flow.
President Barack Obama had been briefed on the new developments, and BP has welcomed the offer of assistance from the defence department to help contain the spill.
Seventy vessels – oil skimmers, tugboats barges and special recovery boats that separate oil from water – as well as five aeroplanes, were working to spray dispersants and round up oil, BP said.
Eleven workers are missing and presumed dead after the worst oil rig disaster in almost a decade. Louisiana Governor Bobby Jindal said the top priority was “to protect our citizens and the environment”.
With the spill moving towards Louisiana’s coast, which contains some 40% of the nation’s wetlands and spawning grounds for countless fish and birds, it was hoped a “controlled burn” of oil contained by special booms would limit the impact.
Environmental experts say animals nearby might be affected by toxic fumes, but perhaps not as much as if they were coated in oil.
On Wednesday afternoon, BP and coastguard boats swept the thickest concentrations of oil into a fire-resistant boom.
This was then towed to a five-mile “burn zone” set up inside the slick, where it was set alight shortly before nightfall.