An offshore oil rig where fire raged for more than 24 hours following a massive explosion, has sunk in the Gulf of Mexico.
The rig's sinking on Thursday raised fears of a major oil spill and dimmed hopes of finding alive 11 workers who are still missing after the blast.
There are fears that up to 1.2 million litres of crude oil a day could pour into the water after the sinking of the Deepwater Horizon, a spill that could reach the coast of the US state of Louisiana, about 80km away.
The rig was doing exploratory drilling off Louisiana when the explosion and fire occurred, sending a column of black smoke hundreds of metres in the air.
Oil giant BP, which contracted the rig, said it has mobilised four aircraft that can spread chemicals to break up the oil and 32 vessels, including a big storage barge to suck up oil from the water.
"It certainly has the potential to be a major spill," Dave Rainey, BP's vice-president for the Gulf of Mexico, said.
Luann White, the director of Tulane's Center for Applied Environmental Public Health, said that were any slick to reach the Louisiana coast it would threaten wild birds, shrimp breeding grounds and oyster beds.
"This latest incident should give the administration and our fellow members of congress pause in their effort to expand oil drilling along the East Coast"
Robert Menendez and Frank Lautenberg, Democratic Party senators
"All of south Louisiana is a very environmentally-sensitive area. It has been stressed because of recent hurricanes and coastal erosion," she told the AFP news agency.
Crews searching for the missing workers, meanwhile, have covered a 5,025 sq km area by air 12 times and by boat five times.
"We do continue with search and rescue activities," Rear Admiral Mary Landry, the 8th District Coast Guard commander, said.
"As time passes, however, the probability of success in locating the 11 missing persons decreases."
At least one relative said family members had been told it was unlikely any of the missing survived Tuesday night's blast.
"They're assuming all those men who were on the platform are dead," Carolyn Kemp, whose grandson, Roy Wyatt Kemp, would have been on the drilling platform when it exploded, said. "That's the last we've heard."
Switzerland-based Transocean, the world's largest offshore drilling contractor, said some of the missing may not have been able to escape the platform.
"Based on reports of crew members, at the time of the incident, they believe they may have been on board and not able to evacuate," Adrian Rose, a vice-president of Transocean, said.
More than 100 workers escaped the explosion and fire, but four were critically injured.
The explosion and sinking have underscored concerns about the environmental impact of plans, announced by Barack Obama, the US president, to expand oil drilling off the country's coasts.
"'Big Oil' has perpetuated a dangerous myth that coastline oil drilling is a completely safe endeavour, but accidents like this are a sober reminder just how far that is from the truth," Democratic senators Robert Menendez and Frank Lautenberg, said in a joint statement on Thursday.
"The bottom line is that when you drill for oil, there is always a risk that not only puts lives on the line, but a risk that puts miles of coastline and the economy on the line as well.
"This latest incident should give the administration and our fellow members of congress pause in their effort to expand oil drilling along the East Coast."
Since 2001, there have been 69 offshore deaths, 1,349 injuries and 858 fires and explosions in the Gulf of Mexico, according to the Minerals Management Service.