An image from a video released Wednesday shows equipment being used to try to plug the gushing oil well 1,500 metres below the water's surface in the Gulf of Mexico. (BP)

BP has resumed its "top kill" procedure of shooting heavy drilling mud into its blown-out oil well in the Gulf of Mexico after a temporary pause in operations to monitor the work.

The suspension also allowed crews to bring in more heavy mud, which BP began pumping into the leaking well 1,500 metres underwater on Wednesday afternoon.

The company has indicated it might take 24 to 36 hours for the procedure to work.

"The fact that it's taken more than 24 hours is not a big surprise," BP chief operating officer Doug Suttles said. "We'll stay at this until we're successful or we determine we can't be successful."

He said crews may also shoot assorted junk such as golf balls and rubber scraps into a piece of equipment known as a blowout preventer to fill holes.

The "top kill" procedure has been successful in the past but has never been attempted 1,500 metres below the water's surface, where the wellhead lies.

In the top kill effort to plug the lead, BP is using the red pipe to pump heavy mud through two separate pipes into the gushing oil well. BP is hoping to pump enough mud into the well to counter the pressure of the leaking oil and push it back down toward its source. (CBC)

If BP succeeds in overcoming the gushing oil with the heavy mud, it will then inject cement into the well to seal it.

Worst spill in U.S history

Preliminary studies suggest the Gulf of Mexico oil spill could be the worst in American history, the head of the U.S. Geological Survey says.

New oil-flow estimates by scientists studying the blown-out well determined it has spilled between 64 million and 148 million litres, far more than the 42 million litres that spilled in the Exxon Valdez disaster, according to survey director Dr. Marcia McNutt.

Early estimates used satellite images to suggest oil was gushing out of a broken pipe rising from the wellhead at a rate of 800,000 litres a day.

On Thursday, scientists with the geological survey said the amount was five times greater.

Earlier Thursday, U.S. President Barack Obama defended his administration's response to the spill, calling it the "single most important thing" it needs to get right.

U.S. President Barack Obama, speaking at a news conference at the White House on Thursday, called the oil spill an unprecedented disaster. (Charles Dharapak/Associated Press)

Obama defends response

"Those who think we were either slow in our response or lacked urgency don't know the facts," Obama told reporters at a White House news conference on Thursday, more than five weeks after the spill began when an offshore rig exploded.

"This entire White House and this entire federal government has been singularly focused on 'How do we stop this leak?' and 'How do we … mitigate the damage to our coastlines?'" he said.

The administration has been facing growing frustration on the part of residents and businesses in the Gulf, who say it hasn't been doing everything it can to stop the leak, contain it and oversee cleanup since the April 20 explosion.

"We can always do better," Obama acknowledged.

"It's going to be entirely possible in an operation this large that mistakes are made [and] judgments prove to be wrong" but the administration maintains "a constant sense of urgency" about the crisis, he said.

Obama blamed BP for not being "fully forthcoming" in providing information to help estimate the size of the spill. BP did not disclose immediately that it had a camera at the well site and did not share images from it until pressured to do so by government officials.

"This is an area where I do think our efforts fell short," Obama acknowledged. "It took too long for us to set up our flow-tracking group that has now made these more accurate ranges of calculation.

"That didn't change our response," he said.

News also surfaced Thursday that the head of the U.S. Minerals Management Service (MMS), which is responsible for offshore drilling, had resigned.

Liz Birnbaum, who has resigned as director of the Minerals Management Service, testifies at a committee hearing on Capitol Hill in Washington on May 18. (Pablo Martinez Monsivais/Associated Press)

Department head resigns

Liz Birnbaum had run the service since July 2009. Her department had faced intense scrutiny in recent days after a report by the Interior Department said MMS staffers had accepted tickets to sports events, lunches and other gifts from oil and gas companies.

The report "can only be described as appalling," Obama said Thursday. He also accused her office of having a "scandalously close relationship" with oil companies.

The Gulf of Mexico spill has already affected more than 160 kilometres of Louisiana coastline, of which 56 kilometres will need to be cleaned up, Suttles said. Another six hectares of marshland will also need to be cleaned up, he said.

The area of polluted marshland has not changed in recent days, he added, but warned that could change at any moment.

With files from The Associated Press