The slick has now grown to about 1,500 sq km
There are fears of an environmental disaster in the Gulf of Mexico, as efforts to clear up an oil spill have been suspended because of bad weather.
A drilling rig leased by the oil company BP exploded and sank off the Louisiana coast last week.
Some 1,000 barrels of oil a day are leaking into the sea from the damaged well, officials say.
They say the oil leak has the potential to damage beaches, barrier islands and wetlands across the coastline.
Eleven workers are still missing and presumed to have been killed in the accident. The search for them has been called off.
More than 100 other workers were rescued.
The Deepwater Horizon had been burning for 36 hours when it sank on Thursday in 5,000 ft (1,500m) of water, despite efforts to control the flames.
It was carrying out exploratory drilling 84km (52 miles) south-east of Venice, Louisiana when the blast occurred.
'Highly complex task'
Bad weather caused cleanup efforts to be suspended over the weekend, allowing the slick to grow to about 580 sq miles (1,500 sq km), officials say.
Oil leaks from a sunken drill pipe 5,000 ft beneath the ocean surface
BP has been using a robot submarine to try to activate a blowout preventer - a series of pipes and valves that could stop the leak.
However, this was a "highly complex task" and "it may not be successful", chief operating officer of BP's exploration and production unit Doug Suttles was quoted as saying by Reuters.
The company has also brought in more than 30 cleanup vessels and several aircraft to spray dispersant on the floating oil.
At the moment, the weather conditions are keeping the oil away from the coastline and it is hoped the waves will break up the heavy crude oil, allowing it to harden and sink back to the ocean floor.
Oil platform blaze off Louisiana
'Number one priority'
The coastguard earlier said it had thought it was dealing only with a surface residual oil spill from the rig.
"In addition to that, is oil emanating from the well. It is a big change from yesterday... This is a very serious spill, absolutely, " said Rear Adm Mary Landry.
In 2009, BP PLC was fined a record $87m for failing to improve safety conditions following a massive explosion that killed 15 people at its Texas City refinery.
But the US Mineral Management Services found no violations on the Deepwater Horizon rig when it carried out routine inspections in February, March and April this year.
President Barack Obama said on Thursday that the government was providing "all assistance needed" for both the rescue and clean up efforts in the troubled area.
He described the crisis on the BP-leased rig as his administration's "number one priority".
No cause for the blast has yet been identified.