The deadly billion-dollar disaster

• President Obama yesterday visited the Louisiana coast. Picture: Getty

The announcement came as President Barack Obama visited the Louisiana coast, where oil has reached important wetlands, closed down the lucrative fishing trade and angered locals whose communities are still recovering from Hurricane Katrina in 2005.

BP chief executive Tony Hayward said the so-called "top kill" procedure, in which heavy drilling mud is pumped into the seabed well-shaft, was showing some signs of success in choking off the leak that has already spewed millions of gallons of oil into the Gulf of Mexico.

But the ultimate success of the operation, never attempted at such depths, was still uncertain.

It could be another 48 hours before BP knows whether it has been successful, he said.

"We don't know whether we will be able to overcome the well," Mr Hayward said. The energy giant was last night maintaining its assessment that the top-kill plugging operation had a 60 to 70 per cent chance of success.

BP shares were down around 5 per cent amid uncertainty over the effort to plug the well.

BP said that the cost of the disaster so far was $930m. However, the cost is sure to multiply with the clean-up of the oil-hit coast. The disaster is now larger than the spill from the Exxon Valdez off the Alaskan coast in 1989.

"This is clearly an environmental catastrophe. There are no two ways about it," Mr Hayward said, reversing previous comments in which he had played down the ecological impact.

The spill has also become a major challenge for Mr Obama.

Opinion polls show many Americans are dissatisfied with his handling of the five-week-old crisis. He was put on the defensive at a news conference earlier this week, rebutting criticism that his administration had been too slow to act and too quick to believe what it was being told by BP.

On his visit to the Gulf coast, Mr Obama inspected oil-trapping booms at a beach in Port Fourchon, the hub of the Gulf oil industry and one of the areas worst affected by crude coming ashore from the spreading spill.

"Obviously, the concern is that until we stop the flow, we've got problems," said Mr Obama, picking up several tar balls from the beach.

Mr Hayward said BP engineers had injected a "junk shot" of heavier blocking materials, such as pieces of rubber, into the ruptured wellhead. They were due to pump in more heavy fluids as part of the top-kill procedure last night.

"We have some indications of partial bridging which is good news," Mr Hayward said. "I think it's probably 48 hours before we have a conclusive view."

Thad Allen, a coastguard admiral, who is leading the oil-spill response, said: "We're very encouraged by the fact that they're able to push the mud down.

"The real question is, can we sustain it, and that will be the critical issue going through the next 12 to 18 hours."

Yesterday's trip was Mr Obama's second to the Gulf since an explosion on the rig killed 11 workers and unleashed the oil from a well-head a mile (1.6 km) under sea.

His tour comes a day after he vowed to "get this fixed" as criticism swelled over what many Americans see as a slow government response to one of the country's biggest environmental disasters.

Mr Obama's predecessor, George W Bush, was heavily criticised for his administration's handling of Hurricane Katrina and Mr Obama is anxious to avoid comparisons.

But the spill could turn into a major political liability before November elections.

Gulf slick could worsen as oil plume discovered

SCIENTISTS have warned that the Gulf of Mexico slick could get much worse after they discovered a 22-mile wide "bubble" of underwater oil.

The discovery, made by researchers on the University of South Florida College of Marine Science's Weatherbird II vessel, is the second significant undersea plume reported since the Deepwater Horizon rig exploded on 20 April, killing 11 workers.

The cloud was nearing a large underwater canyon whose currents fuel the foodchain in Gulf waters off Florida and could potentially wash the tiny plants and animals that feed larger organisms in a stew of toxic chemicals.

Larry McKinney, of the Harte Research Institute for Gulf of Mexico Studies, Texas, said the oil could sweep down the west coast of Florida as a toxic soup as far as the Keys. The plume was detected from just beneath the surface down to about 3,300ft.