ROBERT, La. -- BP admitted defeat Saturday in its attempt to plug the Gulf of Mexico oil leak by pumping mud into a busted well but is readying yet another approach after repeated failures to stop the crude that's fouling marshland and beaches.

BP PLC Chief Operating Officer Doug Suttles said the company determined the "top kill" had failed after it spent three days pumping heavy drilling mud into the crippled well 5,000 feet underwater. More than 1.2 million gallons of mud was used, but most of it escaped out of the damaged riser.

In the six weeks since the spill began, the company has failed in each attempt to stop the gusher, as estimates of how much oil is leaking grow more dire. The spill is the worst in U.S. history -- exceeding even the 1989 Exxon Valdez disaster -- and dumping between 18 million and 40 million gallons of oil into the Gulf, according to government estimates.

"This scares everybody, the fact that we can't make this well stop flowing, the fact that we haven't succeeded so far," Suttles said. "Many of the things we're trying have been done on the surface before, but have never been tried at 5,000 feet."

The company failed in the days after the spill to use robot submarines to close valves on the massive blowout preventer atop the damaged well, then two weeks later ice-like crystals clogged a 100-ton box the company tried placing over the leak. Last week, engineers removed a mile-long siphon tube after it sucked up a disappointing 900,000 gallons of oil from the gusher.

Frustration has grown as drifting oil closes beaches and washes up in sensitive marshland. The damage is underscored by images of pelicans and their eggs coated in oil. Below the surface, oyster beds and shrimp nurseries face certain death.

President Barack Obama said Saturday that the failure of BP's latest effort to stop the damaging flow is "as enraging as it is heartbreaking."

"As I said yesterday, every day that this leak continues is an assault on the people of the Gulf Coast region, their livelihoods, and the natural bounty that belongs to all of us," Obama said. "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 discussed the situation Saturday with Coast Guard Adm. Thad Allen, who is overseeing the response to the spill, and Energy Secretary Steven Chu, Interior Secretary Ken Salazar, Environmental Protection Agency Administrator Lisa Jackson, and senior White House advisers John Brennan and Carol Browner.

Suttles said BP already is preparing for the next attempt to stop the leak that began after the Deepwater Horizon drilling rig exploded in April, killing 11 people.

The company plans to use robot submarines to cut off the damaged riser from which the oil is leaking, and then try to cap it with a containment valve. The effort is expected to take between four and seven days.

"We're confident the job will work but obviously we can't guarantee success," Suttles said of the new plan, declining to handicap the likelihood it will work.

He said that cutting off the damaged riser isn't expected to cause the flow rate of leaking oil to increase significantly.

Obama said that approach next is risky and hasn't been tried before at a depth of 5,000 feet.

The president pledged anew to pursue "any and all responsible means" of stopping the leak until BP completes the drilling of a relief well. But while the relief well is a permanent solution to the problem, BP says it won't be ready until August.

Experts have said that a bend in the damaged riser likely was restricting the flow of oil somewhat, so slicing it off and installing a new containment valve is risky.

"If they can't get that valve on, things will get much worse," said Philip W. Johnson, an engineering professor at the University of Alabama.

Johnson said he thinks BP can succeed with the valve, but added: "It's a scary proposition."

Word that the "top kill" had failed hit hard in fishing communities along Louisiana's coast.

"Everybody's starting to realize this summer's lost. And our whole lifestyle might be lost," said Michael Ballay, the 59-year-old manager of the Cypress Cove Marina in Venice, La., near where oil first made landfall in large quantities almost two weeks ago.

Johnny Nunez, owner of Fishing Magician Charters in Shell Beach, La., said the spill is hurting his business during what's normally the best time of year -- and there's no end in sight.

"If fishing's bad for five years, I'll be 60 years old. I'll be done for," he said after watching BP's televised announcement.