Ken Salazar, the interior secretary, said on Sunday that the administration would keep up its pressure on the oil company.

"Our job basically is to keep the boot on the neck of British Petroleum to carry out the responsibilities they have both under the law and contractually to move forward and stop this spill," he told CNN's "State of the Union" programme.

Wildlife threatened

Government data on Sunday showed that the thickest part of the huge slick had turned northward towards the remote Chandeleur Islands, a chain of uninhabited islets in eastern Louisiana that is an important wildlife area.

in depth

  BP defends clean-up effort
  Spill threatens wildlife
  Blog: 'They saw it coming!'
  US oil spill explained
  Fears grow over oil spill disaster
  Oil spill threatens US coastline
  US fights Gulf oil spill
  Military to quell oil spill
  How the spill happened
  Environmental crisis looms
  Counting the cost:
  Oil exploration

The chain of uninhabited islets in eastern Louisiana is prime marsh and wildlife area, but officials said confirmation of any impacts would not be known until an overflight was conducted.

"Basically what it's showing is that the light sheen is impacting the islands in the Chandeleur Sound," Petty Officer Matthew Schofield of the coast guard told the AFP news agency.

"What we need to do is get an overflight to confirm that we have a light impact. These are just predictions of oil impacts, we have not been able to confirm them."

The Chandeleur Islands form the easternmost point of Louisiana and are part of the Breton National Wildlife Refuge, the second oldest refuge in the United States and home to countless endangered brown pelican, least tern, and piping plover.

Recovery workers have been deploying miles of inflatable booms hoping to contain the oil, but strong winds and heavy seas have caused several of the barriers to break loose, washing them onto the shore.

Chemical dispersant is being used underwater in an attempt to tackle the oil as it leaves the broken riser, while remote-controlled subs were trying to activate a "blow out preventer" – a massive undersea valve that should have blocked oil from flowing into the sea when the rig sank.

With efforts to plug the leak so far unsuccessful and worsening weather in the coming days likely to further hamper attempts to plug the leak, fears are growing of an ecological disaster that could rival the 1989 Exxon Valdez tanker spill in Alaska.

That disaster spilled an estimated 40.9 million litres of crude oil into the previously environmentally pristine waters of Prince William Sound, devastating local wildlife.