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 today or the weekend before the company knows whether the procedure known as a top kill has cut off the oil that has been flowing for five weeks.

President Barack Obama looks through a doorway before the start of a news conference Thursday in the East Room of the White House in Washington. Obama acknowledged his administration could have done better in dealing with the biggest oil spill in the nation’s history.

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 2 1/2 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.”

New plume

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.