METAIRIE, La. -- BP sliced off a pipe with giant shears Thursday in the latest bid to curtail the worst spill in U.S. history, but the cut was jagged and placing a cap over the gusher will now be more challenging, Coast Guard Adm. Thad Allen said.

BP turned to the shears after a diamond-tipped saw became stuck in the pipe halfway through the job, yet another frustrating delay in six weeks of the Gulf of Mexico spill.

The cap will be lowered and sealed over the next couple of hours, Allen said. It won't be known how much oil BP can siphon to a tanker on the surface until the cap is fitted, but the irregular cut means it won't fit as snug as officials had hoped.

"We'll have to see when we get the containment cap on it just how effective it is," Allen said. "It will be a test and adapt phase as we move ahead, but it's a significant step forward."

Even if it works, BP engineers expect oil to continue leaking into the ocean.

The next chance to stop the flow won't come until two relief wells meant to plug the reservoir for good are finished in August.

This latest attempt to control the spill, the so-called cut-and-cap method, is considered risky because slicing away a section of the 20-inch-wide riser could remove kinks in the pipe and temporarily increase the flow of oil by as much as 20 percent.

Live video footage showed oil spewing out of the top of the blowout preventer, but Allen said it was unclear whether the flow had increased.

"I don't think we'll know until the containment cap is seated on there," he said. "We'll have to wait and see."

President Barack Obama will return to the Louisiana coast Friday to assess the latest efforts, his third trip to the region since the April 20 disaster. It's also his second visit in a week.

BP's top executive acknowledged Thursday the global oil giant was unprepared to fight a catastrophic deepwater oil spill. Chief executive Tony Hayward told The Financial Times it was "an entirely fair criticism" to say the company had not been fully prepared for a deepwater oil leak. Hayward called it "low-probability, high-impact" accident.

"What is undoubtedly true is that we did not have the tools you would want in your tool-kit," Hayward said in an interview published in Thursday's edition of the London-based newspaper.

Oil drifted six miles from the Florida Panhandle's popular sugar-white beaches, and crews on the mainland were doing everything possible to limit the catastrophe.

The Coast Guard's Allen directed BP to pay for five additional sand barrier projects in Louisiana. BP said Thursday the project will cost it about $360 million, on top of about $990 million it had spent on response and clean up, grants to four Gulf coast states and claims from people and companies hurt by the spill.

As the edge of the slick drifted toward Pensacola's beaches, emergency workers rushed to link the last in a miles-long chain of booms designed to fend off the oil. They were slowed by thunderstorms and wind before the weather cleared in the afternoon.

Forecasters said the oil would probably wash up by Friday, threatening a delicate network of islands, bays and white-sand beaches that are a haven for wildlife and a major tourist destination dubbed the Redneck Riviera.

"We are doing what we can do, but we cannot change what has happened," said John Dosh, emergency director for Escambia County, which includes Pensacola.