The US Coast Guard says five times as much oil as previously thought could be leaking from a well beneath where a rig sank in the Gulf of Mexico last week.
Rear Admiral Mary Landry said 5,000 barrels (210,000 gallons) a day were now thought to be gushing into the sea 50 miles (80km) off Louisiana’s coast.
A third leak had also been discovered at the site, Adm Landry said.
Earlier, a Coast Guard crew 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, officials said.
Weather forecasters have meanwhile warned that changing winds could drive the oil slick ashore by Friday night.
Adm Landry said experts from the National Oceanic Atmospheric Administration (NOAA) had revised up their estimate for the leak based on aerial surveys, applying dispersants, studying the trajectory of the slick, local weather conditions, and other factors.
“This is not an exact science when we estimate the amount of oil. However, the NOAA is telling me now they’d prefer we use at least 5,000 barrels a day,” she told reporters in New Orleans.
Adm Landry also said she had been told of “a new location of an additional breach in the riser of the deep underwater well”, about 5,000ft (1,525m) under the surface.
President Barack Obama had been briefed on the new developments, and the government had offered to have the defence department help contain the spill, she added.
Louisiana Governor Bobby Jindal has requested emergency assistance from the federal government.
“Our top priority is to protect our citizens and the environment. These resources are critical to mitigating the impact of the oil spill on our coast,” he said in a statement.
The oil slick currently has a circumference of about 600 miles (970km) and covers about 28,600 sq miles (74,100 sq km). Its leading edge is now only 20 miles (32km) east of the mouth of the Mississippi.
The first of the leaks causing it were found on Saturday, four days after the Deepwater Horizon platform, to which the pipe was attached, exploded and sank.
Eleven workers are missing and presumed dead after the worst oil rig disaster in almost a decade.
Adm Landry, who is in charge of the clean-up effort, warned on Tuesday that work on sealing the leaking well using robotic submersibles might take months, and that the Coast Guard would attempt to set light to much of the oil.
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, she said a “controlled burn” of oil contained by special booms could limit the impact.
Controlled burns had been done and tested before, and had been shown to be “effective in burning 50 to 95% of oil collected in a fire boom”, she said.
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, two vessels dispatched by the Coast Guard and the British oil company BP – which had hired the sunken rig – swept the thickest concentrations of oil into a fire-resistant boom.
They then towed it to a five-mile “burn zone” set up inside the slick, where it was set alight shortly before nightfall and allowed to burn.
If the test is deemed successful, BP is expected to continue the controlled burns as long as the weather conditions are favourable.
The move came after BP said it had not been able to activate a device known as a blow-out preventer, designed to stop oil flow in an emergency.