After a brief pause to assess its progress Thursday evening, BP resumed efforts to stem an oil spill that government officials are now calling the largest in U.S. history.

The company said it had briefly suspended its "top kill" operation so that it could determine the effectiveness of its latest effort to plug a broken oil well 1,500 metres below the surface, which has been gushing oil and gas for five weeks.

But it may take until Friday night or later to determine whether shooting mud, concrete and other "junk" material into the broken pipe will seal it successfully. BP officials have put the odds of success at 60 to 70 per cent.

Meanwhile, U.S. Geological Survey Director Marcia McNutt announced that scientists have determined that the undersea well is leaking at least twice times as much oil than was originally estimated. But it could be leaking five times as much.

McNutt said two teams of scientists have calculated that the well is spewing between 504,000 and 1 million gallons of oil each day. That's well above the 210,000 gallon-figure that the Coast Guard and BP have previously reported.

That means that at least 19 million gallons -- and perhaps nearly 39 million gallons -- have leaked into the sea since the explosion on the BP-operated Deepwater Horizon oil rig on April 20. That would make it worse than the Exxon Valdez disaster of 1989, when 11 million gallons spilled.

The so-called top kill method is the latest to be adopted by BP, in its campaign to staunch the catastrophic spill. The same technique has been used to stop leaks in above-ground wells. But it has never been tested 1,500 metres underwater -- the same depth where the well is located.

BP said the top kill was working as expected, but admitted that drilling mud was leaking from the broken well.

"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.

Crews are also considering shooting material such as golf balls and pieces of rubber into the leaking pipe in order to plug holes, said BP's chief operating officer, Doug Suttles.

Earlier in the day, a U.S. Coast Guard admiral said the procedure had shut down the flow of hydrocarbons.

While a spokesperson said that BP appreciates such "optimism," the company would not confirm whether it had been successful in stopping up the well, which is located about 80 kilometres off the coast of Louisiana.

There is also a risk that the operation could make the situation worse by causing additional leaks in the undersea well.

Anil Kulkarni, a Penn State mechanical engineering professor, said that if the leak "ruptures all over, then it would be even more difficult to close it."

U.S. President Barack Obama announced new restrictions on U.S. drilling projects Thursday, while the head of the national agency that regulates the industry was forced to resign.

Obama is due to tour the area Friday, a visit that will give him the chance to see the damage that has been done.

Well-known Democratic strategist James Carville said he hoped the sight of the damage to the Louisiana coastline would spur the president to do more.

"I think you're going to see some real action," said the Louisiana-raised Carville.

Deano Bonano, chief of the Homeland Security Department for Jefferson Paris in Louisiana, said local officials were frustrated that Washington hasn't played a larger role in responding to the emergency.

"BP is an oil company, not an emergency response agency," he told CTV News Channel by phone. "They've treated it more like an oil spill cleanup and not a protective measure."

"Our goal from day one was, ‘hey look, the oil is offshore -- let's protect our shoreline. Let's not treat it as a spill, let's keep it from being a spill.'"

With files from The Associated Press