GULF OF MEXICO - BP says they have temporarily suspended pumping mud to stop Gulf oil leak as they assess the situation. BP says it plans to resume pumping mud Thursday night in effort to plug Gulf oil leak.

BP's attempt to choke off the gusher at the bottom of the Gulf of Mexico appeared to be making some progress, officials said Thursday as dire new government estimates showed the disaster has easily eclipsed the Exxon Valdez as the biggest oil spill in U.S. history.

As the world waited to see whether the "top kill" would work, 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 started shooting heavy drilling mud into the blown-out well 5,000 feet underwater on Wednesday afternoon. It was the latest in a string of attempts to stop the oil that has been spewing for five weeks, since the Deepwater Horizon rig exploded and sank off the coast of Louisiana.

Eleven workers were killed in the accident. Coast Guard Adm. Thad Allen said Thursday afternoon that the mud was stopping some oil and gas but BP was still pumping it in. BP said it should know by the end of the day whether it worked. "It's a work in progress," Allen said. "We need to let it play itself out."

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.

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 2½ to five times higher than what BP and the Coast Guard initially 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 nearly 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 spokesman Steve Rinehart said the previous estimate of 210,000 gallons a day was based on the best data available at the time.

As for the new figures, he said: "It does not and will not change the response. We are going all out on our response."

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 last 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 put an end to 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.

Workers on doomed rig: Corners cut to save money

Two workers injured when an oil rig exploded in the Gulf of Mexico tell Congress that the companies in charge of the doomed drilling operation cut corners and neglected maintenance in a race toward higher profits.

Laborer Stephen Stone told the House Judiciary Committee Thursday that the companies gambled with workers' lives. Oil has been leaking in the Gulf since the Deepwater Horizon exploded April 20, killing 11 workers.

But Jimmy Harrell, the Deepwater Horizon's offshore installation manager, told a panel of Coast Guard and Minerals Management Service officials at a separate hearing near New Orleans that he didn't feel pressured at all.

Federal Gulf cleanup costs at $87M and rising

Federal officials say cleaning up the massive oil spill in the Gulf of Mexico has already cost the government $87 million, making it the third-most expensive cleanup effort in the nation's history.

Coast Guard Rear Adm. Mary Landry has distributed that money to state and federal agencies directly involved in the cleanup. Those include the National Oceanic and Atmospheric Administration, which projects the oil slick's trajectory, and the U.S. Fish and Wildlife Service, which rescues oil-soaked birds.

A senior financial analyst at the National Pollution Funds Center says an additional $38 million in emergency money has been assigned to the Deepwater Horizon spill, but it has yet to be spent.

The most expensive cleanup was the Exxon Valdez spill, which cost $121 million. The second was $89 million for cleaning up a 1994 oil spill off the coast of San Juan, Puerto Rico.