BP has launched a two-pronged attempt to plug the blown-out oil well in the Gulf of Mexico using heavy mud and dense rubber balls, but the company's CEO said it won't be clear for 48 hours whether the efforts will stop the massive oil spill.

CEO Tony Hayward said in an interview Friday morning that the "junk shot" method — which pumps rubber and other man-made material into the gushing well — has been added to the "top kill," which has been slowing the flow of oil by pumping mud into the source since Thursday.

Speaking on the same program, U.S. incident commander Thad Allen said the flow of oil and gas from the broken well has been stopped, but that the real test will be whether the stoppage can be sustained.

"The real challenge is to put enough mud into the well to keep the pressure where they can put a cement plug over the top," he said on ABC's Good Morning America.

The next 12 to 18 hours will be "very critical" in the attempt to protect the fragile ecosystem in the Gulf, which is being overwhelmed by the largest oil spill in U.S. history.

Hayward said BP's efforts have been going "pretty well according to plan," and that the company planned to begin pumping more mud into the well again later today.

President Obama arrived Friday in the Gulf, where he planned to tour a beach with Allen and visit with local officials. A day earlier, he 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," he said.

Friday's trip was the president's second to the coast since the BP-leased oil rig Deepwater Horizon exploded April 20, killing 11 workers and triggering the spill.

He will be briefed in Grand Isle, La., where the public beach has been closed since globs of oil started washing up a week ago. Buggie Vegas, who owns Bridge Side Cabins and Marina there, said Obama needs to see the disaster for himself.

"I think he's going to get the message when he comes down and sees how bad it is," Vegas said.

The top kill is oil and gas giant BP PLC's latest untested bid to end a spill that has, by the most conservative estimate, surpassed the Exxon Valdez disaster by gushing more than 18 million gallons into the Gulf. It has worked on land but never been tried 5,000 feet underwater, and Hayward gave it a 60 to 70 percent chance of success.

He said Friday morning the top kill was progressing as planned and BP engineers had completed a second phase by pumping what he called "loss prevention material" into a crippled piece of equipment known as a blowout preventer to form a bridge against which crews could pump more heavyweight mud.

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

If the top kill fails, BP's next best hope for controlling even part of the leak is a tinier version of something that has already been tried: a steel containment box to cap the well. A 100-ton box lies junked on the ocean floor, abandoned by BP after ice-like crystals clogged it.

While BP officials say the smaller box shouldn't have that problem, it's clearly not their preferred method. It has been sitting in reserve on the seabed for more than a week while engineers first tried to siphon off oil through a mile-long tube.

That succeeded in collecting 924,000 gallons before crews took it out to allow room for the top kill attempt.

If the small box doesn't work, BP officials say they will go back to the tube, crossing their fingers that a relief well still weeks away from being drilled far enough to affect the leak will help stop the bleeding.
And even that's not a sure bet. Obama said Thursday that authorities insisted BP drill a second relief well as a backup since such wells often miss their mark.

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

Two teams of scientists calculated the well has been spewing between 504,000 and more than a million gallons a day, which in the worst-case scenario means about 39 million gallons could have leaked.

The 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.

The new Gulf spill estimates released Thursday were far higher than the 210,000 gallons that BP and the Coast Guard had guessed was pouring from the well.

But even the highest estimate would not make the spill 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.

The Associated Press contributed to this report.