In the six weeks since an explosion hit BP's Deepwater Horizon drilling rig, killing 11 workers, the leaking well has spewed some 68 million litres of oil into the Gulf of Mexico.

in depth

The spill is the worst in US history - exceeding even the Exxon Valdez disaster in 1989 off the Alaska coast.

BP has so far spent $940m to try to plug the leak and clean up the sea and soiled coast.

"This scares everybody, the fact that we can't make this well stop flowing, the fact that we haven't succeeded so far," Suttles said.

"Many of the things we're trying have been done on the surface before, but have never been tried at 5,000 feet."

Disappointment

News that the top-kill attempt had failed was met with disappointment in the coastal fishing communities in the US state of Louisiana, where oil first made landfall in large quantities almost two weeks ago.

"Everybody's starting to realise this summer's lost. And our whole lifestyle might be lost," Michael Ballay, manager of the Cypress Cove Marina in the town of Venice, told the Associated Press news agency.

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Al Jazeera's Nick Spicer, reporting from Empire, a town  in Louisiana, said: "The next effort will involve cutting out the wellhead and moving a multi-tonne device called a blowout preventer ... and then have a drill ship send down a long pipe with a nozzel that will suck up the oil coming out.

"BP itself has said that it only be able to get a majority of the oil, not all of it."

The top-kill operation was the latest of several failed attempts to plug the leaking well.

In the days immediately after the explosion, BP engineers tried to use robot submarines to close valves on the massive blowout preventer atop the damaged well.

Two weeks later ice-like crystals clogged a 100-tonne containment box the company tried placing over the leak.

And earlier this week engineers removed a mile-long siphon tube from the broken riser pipe after managed to extract a disappointing 3.4 million litres of oil from the well.

Next attempt

With pressure growing on BP to do more to contain the leak, Suttles said engineers were already preparing for the next attempt which would use robot submarines to cut off the damaged riser from which the oil is leaking.

They would then try to cap it with a containment valve in an operation that is expected to take between four and seven days.

At least 68 million litres of oil have spilled into the Gulf of Mexico [AFP]

"We're confident the job will work but obviously we can't guarantee success," Suttles said.

Cutting off the damaged riser was not expected to cause the flow rate of leaking oil to increase significantly, he added.

However, other experts have warned that the operation is risky because a bend in the damaged riser pipe was likely to be restricting the flow of oil.

"If they can't get that valve on, things will get much worse," Philip Johnson, an engineering professor at the University of Alabama, told the Associated Press.

"It's a scary proposition."

BP engineers have said that a permanent solution to the leak, a relief well currently being drilled, will not be ready until August.