BP has successfully cut an underwater wellpipe using hydraulic shears and will now work to place a containment vessel over the leak, the senior US official overseeing the response said Thursday.

"We do have a cut now," said Admiral Thad Allen, a day after workers abandoned an attempt to cut the pipe with a diamond saw in favor of using giant "scissors."

Allen said the successful cut was a "significant step forward," adding that workers would now seek to cover the leak with a device that could capture spilling oil and siphon it up to a containment ship on the sea surface.

Earlier AFP report follows:

BP scrapped a plan Thursday to use a diamond saw to slice through a ruptured oil wellpipe, and said it would now use shears to cut the pipe in the latest blow to efforts to stop the leak.

The British energy giant has battled unsuccessfully for six weeks to cap or contain the disastrous leak, now the worst in US history, since an April 20 explosion tore through a BP-leased rig in the Gulf of Mexico.

Workers are using robotic submarines working in cold waters a mile down (1,600 meters) on the sea bed in an attempt to stem the flow of oil, which was closing in on Florida beaches Thursday after new areas were closed to fishing.

Admiral Thad Allen, the US official overseeing the response to the spill, said Thursday the plan to use the diamond saw had been scrapped after the saw became stuck.

Speaking on ABC's "Good Morning America," he said workers would now use a pair of giant shears, which he described as "a big pair of scissors, powered by hydraulics," to cut through the riser pipe before putting a containment device in place.

The diamond saw would have produced a cleaner cut, allowing workers to connect a containment vessel. But the shears will produce a rougher cut, forcing the use of a "top hat" device that will collect oil and siphon it up to ships.

Workers "have the top hat containment device over the well head. They'll be able to lower that over the riser as soon as they make the cut. That's connected to a ship on the surface," he said.

Meanwhile, BP, whose stock plunged in value this week, was downgraded Thursday by ratings agency Fitch from "AA+" to "AA" because of the risks from the oil spill, which has cost the company a billion dollars so far.

The latest official projections from the National Oceanic and Atmospheric Administration (NOAA) show the slick is now some seven miles (11 kilometers) off Florida's shores and officials warned it would arrive on pristine tourist beaches imminently.

Florida Governor Charlie Crist warned forecasters "are projecting weathered oil from the leading edge could impact the Florida Panhandle as early as this week, possibly in a day or two."

"We are watchful, we are monitoring the situation, and we will do everything to protect our beautiful state," he told a press conference, warning "thousands of tar balls" could be approaching the state.

Florida would be the fourth state hit by the oil since the explosion ripped through the Deepwater Horizon rig, 50 miles (80 kilometers) off the Louisiana coast, killing 11 workers.

More than 125 miles (200 kilometers) of Louisiana coast have been contaminated, triggering long-term fears for the region's already endangered wildlife.

A University of Miami study meanwhile showed the oil slick's surface area now stretches across 9,435 square miles (24,435 square kilometers) of the Gulf -- triple the size of satellite imagery from May 1.

Experts warn also the vast majority of the oil is contained in vast underwater plumes that cannot be measured by analyzing from above.

US officials closed more than a third of Gulf of Mexico waters, extending a fishing ban to 88,502 square miles (229,219 square kilometers) -- about 37 percent of the Gulf's federal waters.

The NOAA said the closures were precautionary "to ensure that seafood from the Gulf will remain safe for consumers," but the most significant expansion of the closed fishing area included a zone off southwest Florida.

The massive spill has frayed the nerves of local residents, who wonder if their lives will ever return to normal.

"Everybody is so stressed here. We're just sitting here waiting and they're not telling us anything because they don't know. I had four people who came yesterday crying," said Grand Isle restaurant owner Annette Rigaud.

BP's chief operating officer Doug Suttles acknowledged that until the cap is in place the flow of oil would likely increase by as much as 20 percent.

The US government, which has launched a criminal investigation into the disaster, has estimated the flow of oil before the riser was cut away at 12,000 to 19,000 barrels a day -- meaning at least 20 million gallons have already poured into the Gulf.

The Coast Guard meanwhile said BP has been ordered to cover the cost of constructing five additional barrier islands off the Louisiana coast to prevent oil from making landfall.

The company said it would pay the estimated 360 million dollars to build a total of six barrier islands.