BREAKING NEWS: As magnitude of spill emerges, BP continues top-kill effort (LIVE FEED, PHOTOS)

ROBERT, La. - As BP labored for a second day Thursday to choke off the leak at the bottom of the Gulf of Mexico, dire new government estimates showed the disaster has easily eclipsed the Exxon Valdez as the biggest oil spill in U.S. history.

After an 18-hour delay to assess its efforts and bring in more materials, BP resumed pumping heavy drilling mud into the blown-out well 5,000 feet underwater. Officials said it could be late Friday or the weekend before the company knows if the procedure known as a top kill has cut off the oil that has been flowing for five weeks.

A COLLECTION OF COVERAGE

To see Thursday's photos from the cleanup, click here.

For photos from Wednesday's operation, click here.

To see the BP OIL SPILL LIVE FEED, click here. This may be shut off as BP works on the pipe.

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To see the latest photos from our untouched beaches, click here.

As the world waited, President Barack Obama announced major new restrictions on drilling projects, and the head of the federal agency that regulates the industry resigned under pressure, becoming the highest-ranking political casualty of the crisis so far.

BP PLC insisted the top kill was progressing as planned, though the company acknowledged drilling mud was escaping from the broken pipe along with the leaking crude.

"The fact that we had a bunch of mud going up the riser isn't ideal but it's not necessarily indicative of a problem,'' said spokesman Tom Mueller.

Early Thursday, officials said the process was going well, but later in the day they announced pumping had been suspended 16 hours earlier. BP did not characterize the suspension as a setback, and Eric Smith, associate director of the Tulane Energy Institute, said the move did not indicate the top kill had failed.

"The good news is that they pumped in up to 65 barrels a minute and the thing didn't blow apart,'' Smith said. "It's taken the most pressure it needs to see and it's held together.''

The top kill is the latest in a string of attempts to stop the oil that has been spewing since the drilling rig Deepwater Horizon exploded April 20. Eleven workers were killed.

If the procedure works, BP will inject cement into the well to seal it permanently. If it doesn't, the company has a number of backup plans. Either way, crews will continue to drill two relief wells, considered the only surefire way to stop the leak.

A top kill has never been attempted before so deep underwater. BP Chief Operating Officer Doug Suttles said the company is also considering shooting small, dense rubber balls or assorted junk such as golf balls and rubber scraps to stop up a crippled five-story piece of equipment known as a blowout preventer to keep the mud from escaping.

The stakes were higher than ever as public frustration over the spill grew and a team of government scientists said the oil has been flowing at a rate 2ï¾½ to five times higher than what BP and the Coast Guard previously estimated.

Two teams of scientists calculated the well has been spewing between 504,000 and more than a million gallons a day. Even using the most conservative estimate, that means about 18 million gallons have spilled so far. In the worst-case scenario, 39 million gallons have leaked.

That larger figure would be nearly four times the size of the Exxon Valdez disaster, in which a tanker ran aground in Alaska in 1989, spilling nearly 11 million gallons.

"Now we know the true scale of the monster we are fighting in the Gulf,'' said Jeremy Symons, vice president of the National Wildlife Federation. "BP has unleashed an unstoppable force of appalling proportions.''

BP officials said the previous estimate of 210,000 gallons a day was based on the best data available at the time and that the company's response was not tied to the estimate.

"I don't believe at any time we have misled anybody on this,'' Suttles said.

The spill is not the biggest ever in the Gulf. In 1979, a drilling rig in Mexican waters - the Ixtoc I - blew up, releasing 140 million gallons of oil.

In another troubling discovery, marine scientists said they have spotted a huge new plume of what they believe to be oil deep beneath the Gulf, stretching 22 miles from the leaking wellhead northeast toward Mobile Bay, Ala. They fear it could have resulted from using chemicals a mile below the surface to break up the oil.

In Washington, Elizabeth Birnbaum stepped down as director of the Minerals Management Service, a job she had held since July. Her agency has been harshly criticized over lax oversight of drilling and cozy ties with industry.

An internal Interior Department report released earlier this week found that between 2000 and 2008, agency staff members accepted tickets to sports events, lunches and other gifts from oil and gas companies and used government computers to view pornography.

Polls show the public is souring on the administration's handling of the catastrophe, and Obama sought to assure Americans that the government is in control and deflect criticism that his administration has left BP in charge.

"My job right now is just to make sure everybody in the Gulf understands: This is what I wake up to in the morning, and this is what I go to bed at night thinking about. The spill,'' he said.

Obama said he would end the "scandalously close relationship'' between regulators and the oil companies they oversee. He also extended a freeze on new deepwater oil drilling and canceled or delayed proposed lease sales in the waters off Alaska and Virginia and along the Gulf Coast.

Fishermen, hotel and restaurant owners, politicians and residents along the 100-mile stretch of Gulf coast affected by the spill are fed up with BP's failures to stop the spill. Thick oil is coating birds and delicate wetlands in Louisiana.

"I have anxiety attacks,'' said Sarah Rigaud, owner of Sarah's Restaurant in Grand Isle, La., where the beach was closed because blobs of oil that looked like melted chocolate had washed up on shore. "Every day I pray that something happens, that it will be stopped and everybody can get back to normal.''

Charlotte Randolph, president of Louisiana's Lafourche Parish, one of the coastal parishes affected by the spill, said: "I mean, it's wearing on everybody in this coastal region. You see it in people's eyes. You see it. We need to stop the flow.''

"Tourism is dead. Fishing is dead. We're dying a slow death,'' she added.

The Coast Guard approved portions of Louisiana's $350 million plan to ring its coastline with a wall of sand meant to keep out the oil.

Read an earlier version of this story

COVINGTON, La. — An untested procedure to plug the blown-out oil well in the Gulf of Mexico seemed to be working, officials said Thursday, but new estimates from scientists showed the spill has already surpassed the Exxon Valdez as the worst in U.S. history.

A team of scientists trying to determine how much oil has been flowing since the offshore rig Deepwater Horizon exploded April 20 and sank two days later found the rate was more than twice and possibly up to five times as high as previously thought.

The fallout from the spill has stretched all the way to Washington, where the head of the federal agency that oversees offshore drilling resigned under pressure Thursday.

Even using the most conservative estimate, the new numbers mean the leak has grown to nearly 19 million gallons over the past five weeks, surpassing the size of the 1989 Exxon Valdez disaster in Alaska, which at about 11 million gallons had been the nation's worst spill. Under the highest Gulf spill estimate, nearly 39 million gallons may have leaked, enough to fill 30 school gymnasiums.

"Now we know the true scale of the monster we are fighting in the Gulf," said Jeremy Symons, vice president of the National Wildlife Federation. "BP has unleashed an unstoppable force of appalling proportions."

BP did not immediately comment on the new estimate.

U.S. Geological Survey Director Marcia McNutt said two different teams of scientists calculated that the well has been spewing between 504,000 and more than a million gallons a day.

BP and the Coast Guard estimated soon after the explosion that about 210,000 gallons a day was leaking, but scientists who watched underwater video of well had been saying for weeks it was probably more.

Marine scientists also said Thursday they have discovered a massive new plume of what they believe to be oil deep beneath the Gulf, stretching 22 miles from the leaking wellhead northeast toward Mobile Bay, Ala. The discovery by researchers on the University of South Florida College of Marine Science's Weatherbird II vessel is the second significant undersea plume recorded since the rig exploded.

David Hollander, associate professor of chemical oceanography at the school, says the thick plume was detected just beneath the surface down to about 3,300 feet and is more than 6 miles wide. Scientists are worried the plumes may be from chemicals used to break up the oil.

Last week, BP inserted a mile-long tube to siphon some of the oil into a tanker. It sucked up 924,000 gallons, but engineers had to dismantle it so they could start the risky procedure known as a top kill to try to cut off the flow altogether by shooting heavy drilling fluid into the well.

If that works, BP will inject cement into the well to seal it. The top kill has been used above ground but has never been tried 5,000 feet beneath the sea. BP pegged its chance of success at 60 to 70 percent.

Lt. Commander Tony Russell, an aide to Coast Guard Adm. Thad Allen, said Thursday the mud was stopping some oil and gas but had a ways to go before it proved successful. The top kill started Wednesday night and it could be several days before officials know if it is working.

"As you inject your mud into it, it is going to stop some hydrocarbons," Russell said. "That doesn't mean it's successful."

BP spokesman Tom Mueller also discounted news reports that the top kill had worked.

"We appreciate the optimism, but the top kill operation is continuing through the day today — that hasn't changed," he said Thursday morning. "We don't anticipate being able to say anything definitive on that until later today."

In Washington, meanwhile, Minerals Management Service Director Elizabeth Birnbaum stepped down just hours before a planned White House press conference where President Barack Obama was expected to extend a moratorium on new deepwater oil drilling.

Birnbaum and her agency came under withering criticism from lawmakers of both parties over lax oversight of drilling and cozy ties with industry. An internal Interior Department report released earlier this week found that between 2000 and 2008, agency staff members accepted tickets to sports events, lunches and other gifts from oil and gas companies and used government computers to view pornography. Birnbaum had run the service since July 2009.

After receiving the results of a 30-day safety review from Interior Secretary Ken Salazar, Obama also planned to delay controversial lease sales off the coast of Alaska and cancel entirely plans for drilling lease sales in the Western Gulf and off the coast of Virginia, according to a White House aide.

Polls show the public is souring on the administration's handling of the catastrophe, and fishermen, hotel and restaurant owners, politicians and residents along the 100-mile stretch of Gulf coast affected by the spill are fed up with BP's failures to stop the spill. Thick oil is coating birds and delicate wetlands along the Louisiana coast.

"I have anxiety attacks," said Sarah Rigaud, owner of Sarah's Restaurant in Grand Isle, La., where the public beach was closed because blobs of oil that looked like melted chocolate had washed up on shore. "Every day I pray that something happens, that it will be stopped and everybody can get back to normal."

Seven cleanup crew members who reported dizziness, severe headaches and nausea while working in boats off the Louisiana coast remained hospitalized Thursday. The Coast Guard pulled commercial fishing boats from cleanup efforts in Breton Sound on Wednesday after workers first reported feeling sick.

If the top kill fails, BP says it has several backup plans, including sealing the well's blowout preventer with a smaller cap, which would contain the oil. An earlier attempt to cap the blowout preventer failed. BP could also try a "junk shot" — shooting golf balls and other debris into the blowout preventer to clog it up — during the top kill process.

The only permanent solution is drilling a second well, but that will take a couple of months. BP plans to go ahead with that even if the top kill works.

Though the spill is now the biggest in U.S. history, it's not the biggest ever in the Gulf. An offshore drilling rig in Mexican waters — the Ixtoc I — blew up in June 1979, releasing 140 million gallons of oil.