There are fears of an environmental disaster in the Gulf of Mexico as efforts to clean up an oil spill continue.
A drilling rig exploded and then sank off the coast of Louisiana last week.
11 people who were working on the rig are still missing, presumed dead.
Some 1,000 barrels of oil a day are now leaking into the sea from the damaged well.
The US Coast Guard has said the leak has the potential to damage beaches, barrier islands and wetlands across the coastline.
British oil giant BP is using robotic underwater vehicles to try to cap the leaking well.
Satellite images showed the slick had spread by 50% in a day, to cover an area of 1,550sq.km, although officials said some 97% of the pollution was just a thin veneer on the sea's surface.
BP has dispatched skimming vessels to mop up the oil leaking from the debris of the Deepwater Horizon rig, which sank on Thursday, two days after a massive explosion left 11 workers missing.
The US Coast Guard, which helped evacuate another 115 to safety after Tuesday’s blast, which shot balls of flame leaping into the night sky, aborted its massive air and sea search on Friday.
Investigations are ongoing into the cause of the accident, which would be the worst in decades on a US offshore platform if the missing men are not found.
