BP CEO Tony Hayward told ABC's George Stephanopoulos that it could be another 2 days before he knows if the "top kill" process worked.

BP, which owns the well and is the largest oil and gas producer in the United States, began injecting mud into the well on Wednesday afternoon in an untested bid to end a spill whose millions of gallons have surpassed the Exxon Valdez disaster. The catastrophe started with an oil rig explosion April 20 that killed 11 workers.

The maneuver, called a top kill, has worked on land but has never been tried in deep water. It comes after BP failed to plug the leak with a blowout preventer, a massive piece of machinery on top of the well that is supposed to be a fail-safe device, and couldn't capture the crude with a mile-long tube or a 100-ton containment box.

BP's Chief Executive Tony Hayward said on the CBS "Early Show" it would be around 48 hours before they know if the mud worked well enough to keep the leak sealed so cement can be poured in as a cap. Hayward said the effort has a 60 to 70 percent chance of success.

Hayward said things were progressing as planned. He said BP engineers had completed a second phase by pumping what he called "loss prevention material" into the blowout preventer to form a bridge against which they could pump more heavyweight mud.

That part of the operation was completed early Friday and appeared to have been partially successful. BP would go back to pumping more mud later Friday, he said.

"Clearly I'm as anxious as everyone in America is to get this thing done," Hayward said.

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.

Obama was scheduled to attend a briefing Friday at the U.S. Coast Guard Station in Grand Isle, La., by Adm. Thad Allen, who is overseeing the response to the spill. It would be his second visit to the region since the disaster began.

At the White House on Thursday, Obama acknowledged that his administration could have done a better job dealing with the spill and that it misjudged the industry's ability to handle a worst-case scenario.

"I take responsibility. It is my job to make sure that everything is done to shut this down," Obama said.

BP has spent $930 million so far responding to the ruptured well, it said in a regulatory filing Friday, including costs for clean-up and prevention work, drilling relief wells, paying grants to Gulf states, damage claims and federal costs. BP says it's too early to quantify other potential costs and liabilities associated with the spill.

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.

This video is from ABC's Good Morning America, broadcast May 28, 2010.

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Article text largely by The Associated Press. Used with permission.