BP has begun a "top kill" operation to try to stop the flood of oil from a sunken rig in the Gulf of Mexico, but it will take at least 24 hours before officials know whether the attempt worked.
Company officials have given the technique, which is untested 1,500 metres underwater, between a 60 and 70 per cent chance of success.
BP spokesperson Steve Rinehart said crews will pump mud into the well for several hours, while CEO Tony Hayward said it could be about 24 hours before officials know if the technique is working.
A top kill involves filling the well with mud in an effort to plug the leak, followed by cement to permanently seal off the well.
The company had been conducting last-minute testing of the equipment early Wednesday morning.
CTV's Washington Bureau Chief Paul Workman said Wednesday that crews will pump the mud through valves in the heavily damaged blowout preventer, which failed to avert the leak.
"They need something like 50,000 barrels of very dense mud and it will be slowly pushed in," Workman told CTV News Channel from Grand Isle, La.
"Apparently they've done it on land, it's a pretty standard procedure, but when you're that deep underwater where the pressure is really immense, it makes it very difficult, there are a lot of complications and a lot of unknowns."
The devastating leak began April 20, after the Deepwater Horizon oil rig exploded and sank off the coast of Louisiana. Eleven workers were killed in the blast.
In the five weeks since, some seven million gallons of oil have spilled into the sea, some of which has washed up along the U.S. coastline, putting wildlife and its coastal habitat at risk. Additionally, the oil coming out of the well appears to be darker than it was only two weeks ago, with some experts concluding that it is now heavier and may be doing more damage.
Mike Miller, CEO of Safety Boss, a company that helps prevent and contain oil well blowouts, called a top kill a "complicated operation" that could build-up pressure at the bottom of the well and split one or more of the other pipes.
According to Miller, officials will know the measure is working if the mud goes straight into the hole. However, if the mud flows out at the source of the leak, crews will be forced to move to another plan, such as a junk shot, which involves shooting debris into the well to plug the leak. They may also try to place another blowout preventer on top of the one that failed.
"Nothing that they're doing has worked before in these kinds of water depths, which is the complicating issue," Miller told CTV News Channel. "If this well was on land, this would be a two-week issue. But 5,000 feet below the surface is just like working in outer space."
As the operation carries on, Louisiana officials and residents are growing impatient with the response to the spill.
While many hold out hope the method could stop the growing environmental disaster, Miller said the only "sure shot" BP has are the relief wells, which won't be finished for another two months.
BP has set up a video feed of the leak site to its website allowing Internet users to watch the procedure online.
With files from The Associated Press and a report from CTV's Washington Bureau Chief Paul Workman
