Traces of oil from the massive spill in the Gulf of Mexico have reached the mouth of the Mississippi River.
The slick is heading towards an environmentally sensitive area which contains birds, marine life and some of America's richest seafood grounds.
By Thursday night, the oil had creeped into South Pass of the river and was lapping at the shoreline.
Booms in place to protect grasslands and sandy beaches are being overwhelmed by 5ft waves of oily water in the choppy seas.
On Thursday, a new oil leak was discovered at the site of the wrecked drilling rig with an estimated five times more oil spilling into the water every day than previously thought, the US Coastguard said.
An official from BP, which leases the rig, said he did not believe the newly-discovered leak had increased the amount of oil spilling into the water beyond earlier estimates.
But coastguard Rear Admiral Mary Landry disagreed with his statement and said she was relying on a new estimate from the National Oceanic Atmospheric Administration.
She said NOAA experts now estimated 5,000 barrels of oil a day were spilling into the gulf from the rig which exploded and sank last week. Officials had estimated the leak for days at 1,000 barrels a day.
