BP's "top kill" operation to plug the ruptured well in the Gulf of Mexico has failed in a stunning setback to efforts to stem the worst oil spill in US history.
BP and federal authorities said Saturday they are now turning to a new strategy to stop the leak, but it will take at least four to seven days before it can be put into place.
At least 20 million gallons are now estimated to have gushed into the ocean since the disaster unfolded five weeks ago, threatening an environmental and economic catastrophe across hundreds of kilometers of the US Gulf Coast.
"After three full days of attempting "˜top kill,' we have been unable to overcome the flow from the well, so we now believe it's time to move on to the next of our options," BP Chief Operations Officer Doug Suttles told a press briefing.
President Barack Obama called the developments "enraging" and "heartbreaking."
Engineers had spent days pumping some 30,000 barrels of heavy drilling fluid into the leaking well head on the ocean floor in a high-pressure bid to smother the gushing crude and ultimately seal the well with cement.
But the effort failed, and when asked specifically why, Suttles had no direct answer.
"We don't know that for certain," he said.
The announcement marks the latest failure for BP, which despite a series of high-tech operations over the past month has appeared powerless to bring the disaster to heel since an explosion on the BP-leased Deepwater Horizon oil rig April 20 which killed 11 workers. The rig sank two days later.
The British energy giant had stressed that "top kill" was the best chance at stopping the leak other than drilling an entirely new relief well, a process that has already begun but is expected to take another two months.
"Obviously, we're very disappointed in today's announcement and I know all of you are anxious to see this well secured," US Coast Guard Rear Admiral Mary Landry told the briefing.
Efforts will now focus on severing the damaged riser pipes that lay crumpled on the ocean floor, then installing a containment device that could capture the leaking oil and syphon it to the surface.
The new containment plan, scheduled to begin next week, is called the "Lower Marine Riser Package Cap (LMRP Cap)."
It is a complex operation that will be carried out by remotely operated robots on the ocean floor, BP officials said nearly one mile (1.6 kilometers) below the spot where the drilling rig exploded.
The robots, wielding cutting tools, will sever the bent riser pipe and replace it with the LMRP cap, BP officials say.
The cap will then be connected to a riser leading to the drill ship Enterprise, nearly above the robots.
Suttle said that even if LMRP works, it will only contain a majority of the oil and not all of it.
The setback came a day after Obama visited the region for the second time since the spill began 40 days ago, in an attempt to bring new urgency to the response.
Obama toured some of the affected areas in Louisiana on Friday and pledged to do whatever it takes to help Americans whose livelihoods have been upended by the spill.
On Saturday, the president said the new approach envisioned by BP was "not without risk" and has never been attempted before at this depth.
But he reiterated his determination to get the situation under control.
"It is as enraging as it is heartbreaking, and we will not relent until this leak is contained, until the waters and shores are cleaned up, and until the people unjustly victimized by this manmade disaster are made whole," Obama said.
He ordered Interior Secretary Ken Salazar and other top environmental officials to return to the region next week to continue their work aggressively responding to the spill.
Since it began, an estimated 12,000 to 19,000 barrels of crude have been gushing into the Gulf each day.
The disaster has already closed stretches of coastal fishing waters, endangering the seafood industry and tourism, and threatening a catastrophe for Louisiana marshes, home to many rare species.
Government data released Thursday suggested between 18.6 million gallons and 29.5 million gallons of oil have poured into the Gulf far more than the roughly 11 million gallons of crude spilled in the 1989 Exxon Valdez disaster.
Amid the environmental catastrophe, there were also growing fears for the health of cleanup workers exposed to the oil and chemical dispersants.
Four more crewmen aboard ships helping burn off surface oil were evacuated to hospital late Friday after falling ill, a day after the Coast Guard announced that seven workers were evacuated for medical emergencies.
Landry said Saturday that the Coast Guard would continue to use subsea dispersants, chemicals which help break up the crude into droplets that can evaporate or biodegrade more quickly, but which critics say could be a health hazard.
On Saturday, Louisiana state officials called on BP to create a US$300-million fund to mitigate the immediate and long-term impact of the spill on businesses and local residents.
State agencies also sent a letter to the British oil giant seeking another 457 million dollars to fund a 20-year seafood safety plan, warning that "the future of this industry is in peril."