BP on Wednesday launched a complex, risky deep sea operation to cap the Gulf of Mexico oil leak, under huge pressure to get it right this time and stop the five-week-old gusher.
Shortly after winning final approval from US officials for the procedure to go ahead, the British energy giant announced the maneuver dubbed a "top kill" had begun at 1800 GMT.
But after several previous failed attempts to cap the oil, BP boss Tony Hayward has already downplayed hopes of success, cautioning such a procedure has never been tried before at such depth and against such pressure.
He warned it was expected to take two days to complete the difficult operation to inject heavy drilling fluids into the oil flow and then seal it with cement. The work is being carried out by robotic submarines a mile (1,600 meters) down on the seabed.
The Deepwater Horizon rig, just 50 miles (80 kilometers) off the Louisiana coast, exploded on April 20, killing 11 workers.
Its fractured pipe has been spouting out oil from deep below the Earth's crust for 36 straight days creating a massive slick washing up along the Louisiana coastline and threatening endangered birds, animals and plants.
"It's clear that this will be a transforming event in the history of deepwater exploration," Hayward admitted as BP faces increasing pressure to stop the spill, after managing to capture some oil via a tube inserted last week.
President Barack Obama vowed Wednesday his administration would not rest until the leak is plugged and the massive clean-up operation complete, calling it a "heartbreaking" disaster.
"We will not rest until this well is shut, the environment is repaired and the cleanup is complete," Obama said, ahead of his second trip to the Gulf Coast on Friday.
Obama, who offered "every resource necessary to put a stop to this thing," is expected to announce stringent new offshore oil regulations at a press conference on Thursday after getting an Interior Department report into the causes of the disaster.
"A lot of damage has been done already livelihoods destroyed, landscapes scarred, wildlife affected, lives have been lost," Obama said.
Millions of gallons of oil has seeped into the waters, contaminating Louisiana's fragile marshes and wetlands, although the exact amount of crude unleashed by the spill is unknown.
The "top kill" involves smothering the leak with heavy drilling fluid then sealing it with concrete, but Hayward has only put the chances of success at 60 to 70 percent.
The new bid to cap the leak comes as BP admitted there had been at least seven failures and warning signs in the hours before the explosion that something was wrong.
"What we're seeing here is a whole series of failures. We've identified... at least seven," said Hayward.
"It's very clear that much more needs to be put in place to deal with this situation should it ever occur again."
A memo released by senior US lawmakers late Tuesday detailed three warning signs in the last hour before the explosion, including sudden, unexpected pressure rises and fluid leaks.
Officials are also readying back-up options but some, including the drilling of relief wells to divert the flow and allow the original well to be capped, could take several months.
Hayward announced meanwhile that BP had rebuilt the "blowout preventer" in the past week -- a special valve which is supposed to automatically shut off a leak, but which failed on the Gulf of Mexico rig leased by BP from Transocean.
Thanks to a live webcam which BP has placed close to the leak, the whole procedure will be seen live on US television, with news channels streaming footage of the black oil plume rising from the broken well pipe.
Explore further: Lights out in Australia as Earth Hour kicks off