Underwater robots are trying again to help control oil gushing from a broken wellhead riser in the Gulf of Mexico, using shears and saws to cut through the leaking pipe to prepare for a containment cap.

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A diagram shows how the top cap fits over the blowout preventer, part of a 'cut and cap' process to contain oil gushing from a broken wellhead riser in the Gulf of Mexico. ((BP))

The robots have already cut through the top of the riser pipe, which lies crumpled on the seabed 1,500 metres below the water's surface.

A key second cut, where the pipe attaches to the blowout preventer, stalled because the diamond wire saw became stuck. Late Wednesday, however, the saw was reported to be have been freed.

If the saw cut is smooth enough, crews will lower a rubber-sealed "top cap" snugly over the well's broken blowout preventer in order to capture the oil and direct it through a pipe to a tanker on the water's surface. Otherwise, they'll lower a "top hat," a wider version of the top cap, to perform the same task.

The process, known as a "cut and cap," is perhaps the riskiest in a string of failed attempts to contain the spill because cutting the pipe means increasing the already massive flow of oil by 20 per cent, at least 375,000 litres more a day.

"It is an engineer's nightmare," said Ed Overton, a Louisiana State University professor of environmental sciences. "They're trying to fit a 21-inch cap over a 20-inch pipe a mile away.

"That's just horrendously hard to do. It's not like you and I standing on the ground pushing — they're using little robots to do this."

Hurricane season adds to threat

Nearly 380 million litres of crude have already spilled into the Gulf, according to worst estimates by BP, which is responsible for the spill cleanup.

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U.S. Coast Guard Admiral Thad Allen, national incident commander, speaks to reporters about procedures to contain the BP oil leak on Tuesday. ((Cheryl Gerber/Associated Press))

Oil has washed up on the shores of Louisiana, 80 kilometres from where a BP-leased drilling rig, the Deepwater Horizon, exploded on April 20 and sank, causing the spill. Some 200 kilometres of coastline have been affected there.

Now the "threat is shifting to Mississippi and Alabama," Allen said. Oil has made contact with barrier islands in Mississippi Sound; tar balls and sheening have been reported in Alabama.

In Florida, officials confirmed an oil sheen about 14 kilometres from the famous white sands of Pensacola Beach. Crews shored up kilometres of boom and prepared for the mess to make landfall as early as Wednesday.

"It's inevitable that we will see it on the beaches," said Keith Wilkins, deputy chief of neighbourhood and community services for Escambia County.

The oil is not contained in a "huge monolithic" slick but "a bunch of smaller oil slicks, some very large … and scattered over a 200-mile [320-kilometre] radius," Allen said Tuesday.

With files from The Associated Press