A slick measuring 48 miles (77 kilometers) by 39 miles at its widest points has developed 30 miles off the ecologically fragile Louisiana coast since the rig sank last week following an explosion that apparently killed 11 workers.
British energy giant BP, which leases the stricken Deepwater Horizon platform, has been using four robotic submarines to try to fully activate the giant 450-tonne blowout preventer and shut off the flow of oil.
But BP officials suggest the unprecedented operation, which is being conducted remotely a mile down on the seabed, is a longshot and admit they may have to resort to drilling relief wells, a process that would take far longer.
"It is possible that it could take two to three months for a relief well to be drilled," Bill Salvin, a spokesman at the joint information center set up by BP and US-based platform contractor Transocean, told AFP.
Salvin also mentioned a "worst-case scenario" that would see recovery teams "lose total control of the well" and caus