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

Two teams of scientists calculated that the well has been spewing between 504,000 and more than 1 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.

BP announced late in the day that it had suspended shooting heavy drilling mud into the blown-out well 5,000 feet underwater so it could assess how the effort was working and to bring in more materials.

Thursday evening the company said it had resumed the pumping procedure known as a top kill. Officials said it could be late today or the weekend before the company knows if it has cut off the oil that has been flowing for five weeks.

BP insisted that the top kill was progressing as planned, though the company acknowledged that 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 that pumping had been suspended around midnight Wednesday. 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 that 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 never has been attempted so deep underwater. BP Chief Operating Officer Doug Suttles said the company also is 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 2 1/2 to five times higher than what BP and the Coast Guard previously estimated.

"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 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 think to be oil deep beneath the Gulf, stretching 22 miles from the leaking wellhead northeast toward Mobile Bay, Ala.

They fear that it could have resulted from using chemicals a mile below the surface to break up the oil.

Fishermen, hotel and restaurant owners, politicians and residents along the 100-mile stretch of Gulf coast affected by the spill have said they 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."