A top executive of BP PLC said Sunday there was "no certainty" that the company's latest fallback attempt to control the Gulf of Mexico oil spill would work.

"We're all disappointed the top kill didn't work," Bob Dudley, managing director of BP, said on "Fox News Sunday." He was referring to the failure of BP's effort to stem the oil by pumping heavy drilling liquids into the well.

The company now plans to try to contain the flow of oil from the leak with a "lower marine riser package," or cap. The operation would involve removing a broken drilling pipe, or riser, that lies atop the blowout preventer and capping the valve with a siphon that would take the oil to the surface. Mr. Dudley said "there is no certainty" that the operation—which BP officials say has never been carried out in 5,000 feet of water—would work.

"The question is how much of the oil" the company will be able to contain, Mr. Dudley said. He said he didn't believe the operation would make the spill worse.

Meanwhile, a top aide to President Barack Obama said the administration was preparing for the prospect that none of the measures BP is pursuing, other than relief wells that are several months away, will stop or contain the oil gushing into the Gulf.