He said BP was also considering other methods to capture the flow.

About 400 metres away is the wreckage of the drilling rig Deepwater Horizon, which BP was leasing when it exploded on April 20, blowing open the well and triggering a major environmental crisis.

Eleven workers were killed in the accident at the platform, which sank two days later.

IN DEPTH

An estimated 800,000 litres of oil have been spewing daily ever since, in the biggest oil spill in the US since the Exxon Valdez disaster in Alaska in 1989.

The containment box had been considered the best hope of stemming the flow in the short term.

Methane bubble

The blowout was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before exploding, according to interviews with rig workers conducted during BP's internal investigation.

IN DEPTH

  BP defends clean-up effort
  Spill threatens wildlife
  Blog: 'They saw it coming!'
   Blog: Economic impact
  Videos:
  US battles to protect coast
  US oil spill explained
  Fears grow over oil disaster
  Oil spill threatens coastline
  US fights Gulf oil spill
  How the spill happened
  Counting the cost:
  Oil exploration

While the precise cause is still under investigation, the sequence of events described in the interviews provides the most detailed account of the blast that has poured more than 11 million litres of crude into the Gulf.

According to one interview transcript, a gas cloud covered the rig, causing giant engines on the drill floor to run too fast and explode.

The engines blew off the rig and set "everything on fire", the account said.

John Curry, a BP spokesman, would not comment on whether methane gas or the series of events described in the internal documents caused the accident.

"Clearly, what happened on the Deepwater Horizon was a tragic accident. We anticipate all the facts will come out in a full investigation," Curry said on Friday night.

Possible options

Among the options being considered was to plug the leak by injecting ground-up material in a "junk shot".

"It has certain issues and challenges and risks with it, and that's why we haven't actually progressed up to this point. But we will look and continue to see whether that's a viable option," Suttles, the chief operating officer, said.

Race to stop oil flow

Engineers struggling to find a way to halt the flow of oil from the damaged well are working along three paths:

 Breaking up oil as it emerges from damaged pipes by injecting chemicals into waters near wellhead - early tests show some success, but does nothing to stop flow

 Lowering a 12-metre-high, 98-tonne dome to sea floor where it is hoped it will contain gushing oil and allow it to be pumped to surface and into tankers - never been tried in such deep water

 Drilling two relief wellsseveral miles into ocean floor, where oil could be diverted – could take three months

"It's all to do with we're working in 5,000 feet of water in a very difficult, challenging environment."

Public concern about the oil spill has been focused on the potential environmental and economic damage.

Workers have also sprayed dispersants over the slick to break it up and deployed hundreds of thousands of feet of boom to contain the spreading oil.

But environmentalists have warned that dispersants like Corexit were also dangerous to sea life.

"Those products don't make the oil go away," Joe Griffitt, a marine biologist at Gulf Coast Research Laboratory, said.

"It just falls to the sea bottom. That's where you'll find the sediments and the larvae. So the toxic effect is double."

Suttles said BP had anticipated encountering difficulties with the box, but had not expected them to be as significant as a problem.

Teams were evaluating whether the issue could be overcome by providing heat, methanol or other methods.

The dome had been expected to be operational on Monday and to collect about 85 per cent of the leaking crude by funnelling it up to a barge on the surface.