The unprecedented operation to drop the 100-ton (90-tonne) chamber some 5,000 feet (1,500 meters) below the surface to cap the leak was expected to be completed within hours Friday.

“They are in the process of lowering it now,” BP spokesman John Curry told AFP about the operation seen as the best hope to stave off the biggest US environmental disaster since the 1989 Exxon Valdez spill in Alaska.

A crane lowered the 100-ton dome onto the “Joe Griffin” before the barge embarked on the 12-hour journey from Port Fourchon on the Louisiana coast to the epicenter of the disaster some 50 miles (80 kilometers) offshore.

A white silo with a dome-shaped top that stands five stories high, it carries with it the hopes of coastal communities from Texas to Florida whose very way of life is under threat from the slick.

Days of work off the coast of Louisiana with underwater submarines nearly a mile below the surface finally bore fruit as a valve was secured over the smallest of the three leaks and the flow shut off.

The feat does not alter the overall amount of crude spilling into the sea and threatening the fragile US Gulf coast, but is significant nonetheless as the focus can now narrow on just two remaining leaks.

The operation to place the giant structure over the largest of three oil leaks is unprecedented and, facing depths of almost a mile, remote-controlled submarines will have to guide it into place, hopefully by the weekend.

“We are aiming to put in on the ship today and start the process,” BP spokesman John Curry told AFP, adding that containers for the remaining leaks were still being built.

And the day after President Barack Obama visited the disaster zone on the southern coast of Louisiana and put the blame squarely at the firm’s door, BP accepted full responsibility and accepted to pay the cost.

“BP takes responsibility for responding to the Deepwater Horizon oil spill. We will clean it up,” the British energy giant said, vowing to consider all compensation claims “promptly” and pay them quickly if justified.

Since the explosion and sinking last week of the Deepwater Horizon rig, a disaster scenario has emerged with hundreds of thousands of gallons of crude oil spewing unchecked into the Gulf and moving inexorably northward to the coast.

The spreading black tide threatens wildlife, beaches and one of the world’s most fertile fishing grounds in an area stretching across four states, from Louisiana to Florida.

President Barack Obama, meanwhile, planned to visit the spill area over the weekend to assess the situation first hand, a White House official said.

With up to 200,000 gallons of oil a day spewing into the Gulf of Mexico from a ruptured well, the accident stemming from a sunken offshore rig may soon rival the Exxon Valdez disaster as the worst oil spill in US history.