Workers lowered a huge dome over an oil leak gushing from a sunken rig deep in the Gulf of Mexico yesterday as energy giant BP raced to contain a slick beginning to lap an island off the US coast.

The unprecedented operation to drop the 90-tonne chamber some 1,500 metres below the surface to cap the leak was expected to be completed within hours, after getting under way late Thursday.

"It's about 60 metres above the leak, being lowered very carefully onto the leak," BP's chief executive officer Tony Hayward told CNN.

The operation is seen as the best hope to stave off the biggest US environmental disaster since the 1989 Exxon Valdez spill in Alaska, but officials remain cautious about whether it will work.

"This has never been done in 5,000 feet of water. It's a technology first. It works in three to 400 feet of water. But the pressures and temperatures are very different here," Mr Hayward warned.

"We cannot be confident that it will work."

The British energy company is racing to contain the leak, haemorrhaging some 200,000 gallons of oil a day into the Gulf of Mexico from the sunken Deepwater Horizon rig that exploded on April 20, killing 11 workers.

Once it reaches the seabed, the 12-metre tall white box will be fitted into place by remote-controlled submarines.

BP officials, who leased the rig from Transocean, say they hope to have the dome operational by Monday, when a large part of the oil would then be funnelled up to a containment vessel on the surface for storage and processing.

Oil sheen from the massive crude spill began washing ashore on Thursday on an island off Louisiana, raising fears for the region's fragile ecosystem living in its wetlands and shorelines.

The region is home to vital spawning grounds for fish, shrimp and crabs and a major migratory stop for rare birds.