Crews were working Sunday at the site of a wrecked drilling platform to try to stop a leak that is pouring 1,000 barrels of oil a day into the Gulf of Mexico.
Robot submarines are being used to activate valves at the well head in hopes of cutting off the leak, which threatens the Gulf Coast's fragile ecosystem of shrimp, fish, birds and coral.
The submarine work will take 24 to 36 hours, Doug Suttles, chief operating officer for BP Exploration and Production, said Sunday afternoon.
"I should emphasize this is a highly complex operation being performed at 5,000 feet [1,525 metres] below the surface and it may not be successful," he said.
Oil continued to leak underwater Sunday at the site where the Deepwater Horizon rig exploded on Tuesday. Eleven workers are missing and presumed dead.
For the second consecutive day, high waves prevented boats and equipment from going out to clean the spill. Airplanes sprayed chemicals to break up the oil.
The spill initially appeared to be easily manageable after the oil rig sank Thursday off the Louisiana coast, but it has turned into a more serious environmental problem since the leak was discovered on Saturday.
The oil spill has been growing, with the oily sheen on the surface of the water covering about 1,500 square kilometres on Sunday.
The environmental damage would be especially serious if it reaches land. The spill was still about 115 kilometres from the mainland, but only about 50 kilometres from an important chain of barrier islands known as the Chandeleurs.
The islands, part of a national wildlife refuge, are an important nesting ground for pelicans and other sea birds. They have been under serious threat since Hurricane Katrina washed out much of the sand there.
Suttle said the company is planning to collect leaking oil on the ocean bottom by lowering a large dome to capture the oil and using pipes and hoses to pump it into a vessel on the surface.
"That system has been deployed in shallower water," he said, "but it has never been deployed at 5,000 feet of water, so we have to be careful."
The robot submarines are attempting to close off the flow of oil by activating a shutoff device at the wellhead known as a blowout preventer.
In case that doesn't work, BP PLC, which leased the Deepwater, moved another deepwater rig, the DD3, toward the explosion site. If necessary, the new rig would drill relief wells into the damaged well underneath the ocean floor. That could take several months.
U.S. Coast Guard officials said weather conditions for the next three days would help keep the spill away from the coast.