Frustration

"I'm sure many of you have been watching the plume," Hayward said of the live video stream of the leak.

"All I can say is it is unlikely to give us any real indication of what is going on. Either increases or decreases are not an indicator of either success or failure at this time."

BP's fresh attempt to contain the spill comes as fishermen, hotel and restaurant owners, politicians and residents along the coast become frustrated over the company's previous unsuccessful attempts to stop the leak.

The leak began after an offshore drilling rig exploded on April 20, killing 11 workers.

in depth

By the most conservative estimate, 26 million litres of crude have spilled into the Gulf, fouling Louisiana's marshes, coating birds and other wildlife and curtailing fishing.

Doug Suttles, BP's chief operating officer, said: "We're doing everything we can to bring it to closure, and actually we're executing this top kill job as efficiently and effectively as we can."

BP officials said earlier that the manoeuvre had a 60-70 per cent chance of working.

The US coast guard gave the go-ahead for the move earlier on Wednesday after scientists ran a series of diagnostic tests to determine whether the procedure had a chance of success or if it could potentially make the leak worse.

Next step

A weak spot in the device could blow under the pressure, causing a brand new leak. The mud could also tear a new hole in the leaking well pipe.

John Terrett, Al Jazeera's correspondent in Louisiana, said BP was already planning its next step if the "top-kill' method does not work.

Officials say it may be impossible to clean up affected wetlands and islands [AFP]

"Tony Hayward, the chief executive of BP, appeared on TV this morning. He was already talking about what will happen if this attempt to stop the leak doesn't work," Terrett said.

"Hayward said the next thing they may do is drop what he referred to as a top hat down to try and stop the leak and if that doesn't work there is something called the junk method where they blast golf balls, tires, rope and other bits of trash down into the hole and seal it up with cement.

"The final option is that they are currently boring two more holes into the leaking one and they will eventually siphon the oil off before it gets to the surface. But that's not going to be completed until August.

"However for the oil to be gushing until the middle of August is very damaging indeed."

'Intensively engaged'

The success of the procedure is critical for BP, as well as Barack Obama, the US president.

Obama said on Wednesday that his administration was "intensively engaged" with scientists and engineers to explore all alternative strategies on the disaster.

"We will not rest until this well is shut, the environment is repaired and the cleanup is complete," Obama said ahead of his next trip to the Gulf coast on Friday.

He said the procedure being attempted should greatly reduce or eliminate the flow of crude from the oil spill, if it works.

The president said the "heartbreaking"oil spill underscored the need for alternative fuel sources.