BP has started a "top kill" procedure meant to stop the massive, five-week-old oil leak in the Gulf of Mexico by pumping heavy mud into it.

Engineers hope to pump enough mud into the gusher to overcome the flow of the well. Engineers plan to follow up the mud with cement that the company hopes will permanently seal the well.

"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 chief operating officer Doug Suttles said Wednesday night.

Engineers planned to monitor the well overnight and continue pumping in thousands of litres of the drilling fluid, which is about twice as heavy as water.

"The absence of any news is good news," said coast guard Admiral Thad Allen, who is overseeing the operation. He added: "It's a wait-and-see game here right now, so far nothing unfavourable."

U.S. Interior Secretary Ken Salazar, left, with Deputy Interior Secretary David Hayes, testifies before the House natural resources committee on Capitol Hill in Washington on Wednesday. (Manuel Balce Ceneta/Associated Press)

Allen gave his approval for the procedure after consulting government scientists. The top kill has never before been tried 1,500 metres beneath the surface, and company officials say it could be a couple of days before they know whether it is working.

Earlier Wednesday, BP was analyzing data from a series of diagnostic tests meant to determine if the procedure would be able to stop the flow of oil from the broken well.

The procedure involves pumping enough heavy drilling mud via the blowout preventer (BOP) at the top of the well into pipes connected to it to choke the flow of oil and gas up the well.

To test its chances of success, BP late Tuesday began pumping drilling fluids into the BOP "to measure pressures and validate flow paths," the oil giant said on its website.

"Later on today, I will sit with my team and review the analysis and determine whether or not we should proceed," BP CEO Tony Hayward said Wednesday morning.

"So far it's looking OK, but we haven't got all of the data we need" to decide whether to move forward, he said.

'The fervent hope of everyone is that the top kill effort … will work.' —U.S. Interior Secretary Ken Salazar

If all goes as planned, engineers will pump fluid twice as dense as water from two barges into two 7.6-centimetre-wide lines that will feed it into the BOP.

Crews plan to pump it in at a rate of approximately 7,635 to 9,550 litres per minute in hopes of counteracting the upward pressure of the oil gushing to the surface. They stockpiled 7.9 million litres of the heavy mud, a manufactured substance that resembles clay.

There is no guarantee the top kill will work even if it does go forward. Hayward has pegged its chances of success in this case at 60 per cent to 70 per cent.

"The fervent hope of everyone is that the top kill effort … will work," Interior Secretary Ken Salazar told a U.S. congressional hearing Wednesday.

Those interested in watching the procedure can do so online: the London, England-based company posted live video of the leak — 1,500 metres below the water's surface — on its website a few days ago and has said it will keep it up throughout the procedure.

Hearings continue

The attempt comes as more hearings this week focus on what led to the April 20 explosion aboard the Deepwater Horizon that killed 11 workers and triggered the catastrophe.

Dozens of worker statements describe the hours and minutes before the sudden, violent blowout and many said they were concerned about the pressure coming from below.

Tests within an hour of the blast also indicated pressure was building to abnormal levels, according to a U.S. congressional memo by U.S. Representatives Henry Waxman and Bart Stupak to members of the committee on energy and commerce, which is investigating what went wrong.

Another warning sign was the unexpected loss of fluid from a pipe known as a "riser" five hours before the explosion, which could have indicated a leak in the blowout preventer, the memo said. The blowout preventer is designed to shut down the well in case of an emergency. BP has cited its failure as a contributor to the blast.

Backup plan in works

If the top kill doesn't work, BP plans to try deploying what it's calling the lower marine riser package, or LMRP.

That would require removing the leaking riser pipe from the top of the well to create a flat surface and lowering a containment dome onto it to trap the oil and force it through a pipe to a barge on the water's surface.

The dome is designed to create a seal around its bottom, preventing seawater from leaking into it.

An earlier version of a containment dome, which did not have a seal, failed to work when the seawater inside it mixed with the leaking gas and formed ice-like crystals called hydrates.

The hydrates clogged the main pipe leading out of the containment dome and BP was forced to remove it.

With files from The Associated Press