The US Coast Guard has said five times as much oil as previously¹ thought is leaking from a well beneath where a rig sank in the Gulf² of Mexico last week.

美国海岸警卫队称，墨西哥湾钻井火灾事故发生海域的原油泄露量是预想的五倍之多。

Vessels³ swept some oil into a fire-resistant boom for a "test burn"" width="466" height="280" src="/upimg/100429/4_053539_1.jpg" />
Vessels swept some oil into a fire-resistant boom for a "test burn"

Rear Admiral Mary Landry said 5,000 barrels (210,000 gallons) a day were now thought to be gushing⁴（涌出） 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（水面浮油） , 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 delta⁵（三角洲） , officials said.

Weather forecasters have meanwhile warned that changing winds could drive the oil slick ashore⁶ by Friday night.

President briefed

Adm Landry said experts from the National Oceanic Atmospheric⁷ Administration (NOAA) had revised up their estimate for the leak based on aerial surveys（空中测量） , applying dispersants（分散剂） , studying the trajectory⁸（轨道，轨线） 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 breach⁹ 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（溢出，溅出） , 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 mitigating¹⁰（镇静，减轻） the impact of the oil spill on our coast," he said in a statement.

The oil slick currently has a circumference¹¹ 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（钻井平台） 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 spawning¹² grounds（产卵场） for countless¹³ 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 affected¹⁴ by toxic¹⁵ fumes¹⁶, 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（沉没的，凹陷的） rig - swept the thickest concentrations of oil into a fire-resistant boom.

They then towed（拖） 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 favourable¹⁷.