“They lit it with a little float that has a fuel source on it that floats into the oil and ignites. It did successfully ignite,” Coast Guard petty officer Cory Mendenhall told AFP.
The decision to start burning the slick, which has a 965km circumference, gained even greater import when the US government’s weather service warned that the previously kind winds were about to shift.
“Stronger southeast winds are forecast to persist from Thursday night through Saturday night,” a National Oceanic and Atmospheric Administration forecast chart showed.
“These onshore winds will move floating oil towards the delta with possible shoreline impacts by Friday night.”
If large quantities of the crude, which is leaking from the debris of a rig that sank after a deadly explosion last week, drift into Louisiana’s marshy wetlands, mopping up would be next to impossible.
It would be disastrous for natural parks full of waterfowl and rare wildlife and could also imperil the southern state’s $US2.4 billion-a-year fisheries industry, which produces a significant portion of US seafood.
As miles of inflatable booms were set up to protect the Louisiana coast, Governor Bobby Jindal evoked memories of Hurricane Katrina, which devastated his southern state in August 2005.
“As I’ve said many times before, we must hope for the best and prepare for the worst,” Mr Jindal said, after a flyover of the spill. “We’re approaching this situation just as we would do before a hurricane comes ashore.”
“We’re doing everything we can to protect the livelihood of our citizens who make their living in the fishing industry and the wildlife that grace our coastal areas.”
Oil, at the rate of 42,000 gallons a day, is spewing from the riser pipe that connected the Deepwater Horizon platform to the wellhead before the rig sank last Thursday, two days after a huge explosion that killed 11 workers.
The widow of one of the dead has filed a lawsuit accusing the companies that operated the rig — BP, Transocean and US oil services behemoth Halliburton — of negligence.
The accident has not disrupted offshore gulf oil production, which accounts for more than a quarter of the US energy supply.
BP, which leased the semi-submersible rig from Houston-based contractor Transocean, has been operating four robotic submarines some 1.5km down on the seabed to try to cap the well.
They have failed so far to fully activate a giant 450-tonne valve, called a blowout preventer, that should have shut off the oil as soon as the disaster happened but only partially reduced the flow.
As a back-up, engineers are frantically building a giant dome that could be placed over the leaks to trap the oil, allowing it to be pumped up to container ships on the surface.
Another Transocean drilling rig is also on stand-by to drill two relief wells that could divert the oil flow to new pipes and storage vessels.
But that would take up to three months and the dome is seen as a better interim bet even though engineers need two to four weeks to build it.
Coast Guard Rear Admiral Mary Landry, who is leading the government’s response to the disaster, warned on Tuesday that if BP fails to secure the well it could end up being “one of the most significant oil spills in US history.”