Robotic submersibles are being used to try to stop oil leaking into the Gulf of Mexico from a damaged well almost a mile (1.5km) below the surface.
Some 1,000 barrels (42,000 gallons) of oil a day have been gushing from the well since a drilling rig exploded and sank off the Louisiana coast last week.
British oil company BP, which leased the rig, said the “first-of-its-kind” attempt would take 24 to 36 hours.
A nearby oil platform has been evacuated as a precaution.
Workers on the rig, the Ocean Endeavour, were taken away because the oil slick was coming dangerously close, said the US Minerals Management Service.
Conservation experts say the oil has the potential to damage beaches, barrier islands, wetlands and wildlife reserves along hundreds of miles of coastline in four US states.
Whales have been spotted near the spill but they did not appear to be in distress.
For now, the weather conditions are keeping the oil away from the shore and it is hoped the waves will break up the heavy crude oil, allowing it to harden and sink back to the ocean floor.
The oil is not expected to reach land for at least another three days.
Officials are monitoring the environmental effects by boat and plane.
BP – which is responsible under federal law for the clean-up – said it was using four submersible vehicles, equipped with cameras and remote-controlled arms, to try to activate a blow-out preventer – a series of pipes and valves that could stop the leak. …