BP plugged the smallest of three leaks spewing oil from its well in the Gulf of Mexico but admitted it would have no impact on the giant slick threatening the coasts of five US states.
The leak, at the end of a fractured pipe which used to lead from the well to the sunken Deepwater Horizon rig, was capped with a valve by remote underwater robots operating nearly a mile under water.
It did not reduce the total amount of leaking oil – around 210,000 gallons a day – which will continue to gush out at two other points.
BP attempts to 'dam' Gulf of Mexico oil spill But the success will allow engineers to concentrate on the two other leaks.
A giant 98-ton metal box will be dropped over the largest one within a week.
More than 600 animal species are threatened by the 130-mile wide slick and the Pentagon has authorised the use of 17,500 National Guard troops to assist defensive efforts along the Gulf coastline.
The leading edge of the slick reached land for the first time yesterday with small amounts of oil washing up on beaches in a pristine wildlife reserve on the Chandeleur Islands, 60 miles from New Orleans.
The uninhabited, marshy island chain is home to brown pelicans, which were only recently removed from the endangered species list.
It is the easternmost point of Louisiana and forms part of the Breton National Wildlife Refuge, the second oldest wildlife refuge in the United States.
BP sent a fleet of 22 boats, including 12 shrimp trawlers, to the scene and they began skimming the surface of the slick, putting down protective booms and dropping dispersant chemicals into the oil.
Since the Deepwater Horizon rig exploded and sank two weeks ago, with the loss of 11 lives, it has spilt an estimated 2.5 million gallons of oil.
More than 150,000 gallons of chemicals have been used in attempts to break up the ooze but there are concerns what effect that will have on the environment. Richard Charter, of the Defenders of Wildlife group, said: "It's basically a giant experiment." At the Tri State Bird Rescue centre in Fort Jackson, Louisiana, a brown pelican, rescued from an outlying island, was one of the first oil covered birds to be captured and cleaned. It was dunked in water and detergent and hosed down.
Vet Erica Miller said: "As long as the oil can be kept away from shore that's going to save a tremendous amount of birds." Fifteen miles off the Louisiana coast a sea turtle was seen struggling through the slick. Karla Raettig, of the Coastal Louisiana Restoration organisation, said: "There was nothing we could do at that point to help it.
It was very upsetting." Scientists warned that currents could ultimately take the slick around the bottom of Florida and up towards Miami Beach.