BP has launched a deep sea operation this evening to choke off an oil leak in the Gulf of Mexico.
However, US President Barack Obama has warned that there is no guarantee it would work.
BP is under intense pressure from Mr Obama to bring a swift end to the five-week-old spill that threatens an environmental catastrophe.
Undersea robots are being used to pump heavy fluids and cement down about a mile (1.6km) to the sea-bed well.
The so-called 'top kill' procedure has never been attempted at such depths.
Mr Obama said if successful, BP's plan to cap the well should greatly reduce or eliminate the flow of hundreds of thousands of gallons of crude into the Gulf.
He said: 'We will not rest until this well is shut, the environment is repaired and the cleanup is complete.'
BP Chief Executive Tony Hayward said it would take a day or two to determine whether the top kill procedure had worked.
The oil leak threatens some of the richest fisheries in the US and has already damaged more than 100km of Louisiana's 644km coastline.
The oil has been leaking out since the Deepwater Horizon rig sank last month after an explosion that killed 11 people.
The US Coast Guard approved the top kill operation after US government scientists said it was safe to proceed.
Meanwhile, the trans-Alaska pipeline partly owned by British energy group BP is closed as workers cleaned up a crude oil spill, which flowed into a containment area.
The spill which was estimated at 'several thousand barrels' occurred when power failed during testing of the pump station's fire command system, according to Alyeska Pipeline Service Co, which manages the facility.
'There were no injuries and personnel have been evacuated, the pipeline is currently shut down and an incident management team has been activated to manage the situation.'
Alyeska is owned by a consortium of oil companies, including BP, and the 800 mile trans-Alaska which carries around 10% of daily US oil production.
BP is the company at the centre of the catastrophic Deepwater Horizon spill in the Gulf of Mexico.
