'Top kill' operation fails: BP

BP has scuttled the "top kill" procedure of shooting heavy drilling mud into its blown-out oil well in the Gulf of Mexico after it failed to plug the leak.

BP chief operating officer Doug Suttles told reporters on Saturday that over the last three days, the company has pumped in more than 30,000 barrels of mud and other materials down the well but has not been able to stop the flow.

"These repeated pumping[s], we don’t believe will likely achieve success so at this point it’s time to move to the next option," Suttles said.

He said the decision was made following a significant review by a "brain trust of engineers and scientists" from BP, the industry, and the U.S. Department of Energy and Interior.

He said the company is preparing a new plan that would involve using robot submarines to cut off the source of the leak, a damaged riser, and cap it with a containment valve that's already resting on the sea floor.

The new attempt would take four days to complete, he said.

"We're confident the job will work but obviously we can't guarantee success at this time," Suttles said of the new plan.

Worst spill

BP began pumping heavy mud into the leaking well 1,500 metres underwater on Wednesday afternoon to try to quell the worst oil spill in U.S. history.

The top kill procedure had been successful in the past but had never been attempted 1,500 metres below the water's surface, where the wellhead lies.

It was the latest in a string of attempts to stop the oil that has been spewing since just after the drilling rig Deepwater Horizon exploded April 20, killing 11 workers.

Since Wednesday, BP had pumped huge amounts of mud into the well at a rate of up to 10,200 litres per minute, but it was unclear how much is staying there. A robotic camera on the sea floor appeared to show mud escaping at various times during the operation.

Limited success

BP has also had limited success with trying to shoot assorted junk into a piece of equipment known as a blowout preventer to fill holes to force the mud down the well bore. The company had said the top kill's chance of success was around 60 to 70 per cent. 

BP has said the best way to stop the flow of oil is by drilling relief wells, but those won't be completed until August.

New oil-flow estimates by scientists studying the blown-out well determined it has spilled between 64 million and 148 million litres, far more than the 42 million litres that spilled in the Exxon Valdez disaster.