METAIRIE, La. — BP used giant shears to slice off a pipe this morning in the company's latest bid to contain the Gulf of Mexico oil, but the cut was irregular and placing a cap over the gusher will now be more challenging, Coast Guard Adm. Thad Allen said.

BP had to use the shears after a diamond-tipped saw became stuck in the pipe halfway through the job, yet another frustrating delay in six weeks of failed efforts to stop, or at least curtail, the worst oil spill in U.S. history.

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

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

BP's top executive acknowledged today 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 today's edition of the London-based newspaper.