Crews hit a snag Saturday in the delicate and time-consuming attempt to cap an undersea oil well that is pumping millions of litres of crude into the Gulf of Mexico .

A spokesman for BP said Saturday that a 100-ton steel-and-concrete vault was successfully maneuvered over the ruptured well nearly 1,500 metres below the surface, a critical step in the attempt to seal off the massive leak.

But Doug Suttles, chief operating officer for BP, said methane hydrate ice crystals quickly began to form inside the huge structure, blocking the flow of oil and threatening to lift it off the sea floor.

"As we were placing the dome over the leak source a large volume of hydrates formed inside the top of the dome, requiring us to move the dome to the side of the leak point," Suttles said.

Suttles said workers have moved the four-storey tall vault off the ruptured section of the well and are now trying to figure a way around the problem.

"I wouldn't say it's failed yet," Suttles told a news conference. "I would say that what we were trying to do last night didn't work."

The company also said that a second, larger containment box was being built.

But as BP workers tried to stop the leaking well, more than 150 kilometres away blobs of tar washed up at an Alabama beach full of swimmers.

Authorities in protective gear descended on the public beach on Dauphin Island, five kilometres off the Alabama mainland at the mouth of Mobile Bay and much farther east than oil had been reported.

About a half dozen tar balls had been collected by Saturday afternoon at Dauphin Island, Coast Guard chief warrant officer Adam Wine said in Mobile. Authorities planned to test the substance but strongly suspected it came from the oil spill.

The containment vault, a method never before attempted at such depths, had been considered the best hope of stanching the flow in the short term.

But officials had cautioned that there would be unforeseen challenges during this unprecedented attempt to divert leaking oil.

BP contracted the Deepwater Horizon, the oil rig that sank after an explosion that killed 11 workers two weeks ago. The blast sent more than 757,000 litres of crude a day spilling into the Gulf.

Suttle said the company is also working on other methods of stopping the leak, including having crews drill sideways into the well in hopes of plugging it up with mud and concrete.

The work is becoming increasingly urgent as the spilled oil creeps deeper into the Mississippi Delta.

Crews have been laying booms, spraying chemical dispersants and setting fire to the oil slick to try to keep it from coming ashore.

The U.S. Coast Guard said Saturday that about 7.95 million litres of oil mixed with seawater has been collected since the explosion.

Nearly 190 vessels are involved in the cleanup efforts, laying more than 257 kilometres of boom to contain the oil and using more than 1 million litres of chemicals to break up the oil on the water's surface.

More than 4,500 people are responding and another 2,500 volunteers have been trained to help, the Coast Guard said.

Methane bubble set off blast, report says

The Associated Press reported Saturday that the deadly blowout that set off the massive oil spill was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, bursting through several seals and barriers before exploding.

According to interviews with rig workers conducted during an internal investigation by BP, the well was in the process of converting from conducting exploratory drilling to producing oil when the blast occurred.

Ironically, a group of BP executives was on board the rig celebrating the project's safety record at the time of the explosion, according to the transcripts of the investigation obtained by AP.

While workers were placing concrete seals on the well, deep beneath the surface of the Gulf of Mexico, they encountered a buried pocket of methane, said Robert Bea, a University of California Berkeley engineering professor who was consulted as part of the investigation.

Bea said that the bubble rose up the drill column, expanding as it shot up from the high-pressure deep and breaking through safety barriers as it went.

"A small bubble becomes a really big bubble," he said. "So the expanding bubble becomes like a cannon shooting the gas into your face."

The gas flooded into an adjoining room with exposed ignition sources and exploded.

The BP executives were injured but survived, according to one account. Nine rig crew and two engineers died.