US President Barack Obama has extended restrictions on deep water oil drilling in US coastal waters for another six months in response to the huge oil spill in the Gulf of Mexico.
Mr Obama has also suspended test drilling on 33 rigs in the Gulf of Mexico and has also cancelled the sale of further off-shore leases.
The leak was caused by an explosion on the Deepwater Horizon rig five weeks ago, which also killed 11 workers.
Oil company BP is pumping mud in an attempt to plug the hole in the damaged well.
A BP spokesman said it could be 24 to 48 hours before they can tell if the operation will succeed.
Speaking at the White House, Mr Obama said there was a lesson to be learned from the oil disaster.
Today is a defining day for the energy giant in the five-week oil spill disaster.
Regardless of the outcome of the 'top kill' procedure, the spill is shaping up to be the worst in US history.
The US Geological Survey has estimated that the well is leaking up to 19,000 barrels (798,000 gallons) per day.
BP had estimated the leak at 5,000 barrels (210,000 gallons) per day, but admitted that figure was unreliable.
(Watch live webcam)
If the 'top kill' move works, it will be a first as it has never been done at such depths before.
The technique uses undersea robots to help inject heavy fluids - known as drilling mud - and ultimately cement down about a 1.6km to the sea-bed well to stifle the oil flow.
