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A fleet of specialized vessels and fishing boats is back at sea to resume cleanup of a massive oil slick in the Gulf of Mexico just kilometres off the coast of Louisiana.

Crews also hope to resume deploying chemical dispersants by air.

The boats had been forced to shore on Monday, after high winds and thunderstorms rendered booms positioned to contain the slick almost ineffectual and seafaring dangerous.

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Boats move slowly in the morning fog off the coast of Venice, in southern Louisiana, on Tuesday. ((Alex Brandon/Associated Press))

The winds may also have helped to disperse the slick by forcing some of the oil underwater. The latest satellite image, taken Sunday night, showed a slick measuring about 8,800 square kilometres, or about 40 per cent smaller than it measured last week, according to Hans Graber, a professor in the division of applied marine physics at the University of Miami.

The new image also shows that sizable patches of the slick have broken away and are moving to the north and east, Graber said, or away from Louisiana's shoreline.

Regardless of what the slick looks like from the air, the amount of oil continues to grow. BP crews have been unable to stop the heavy gush of oil — about 800,000 litres (5,000 barrels) a day — from three separate leaks in the well 1,500 metres below the water's surface.

Speaking in Washington, D.C., U.S. President Barack Obama said the oil spill is a reminder that his country "can also face at any time a sudden and costly crisis that can harm our economy."

Restaurant owners, hoteliers and tourism operations along the coastal areas face almost certain losses, as potential visitors delay making reservations during what should be the area's busy booking season.

Two shrimpers from Louisiana have already filed a class-action lawsuit against BP and other parties involved in the drilling operation, seeking $5 million US in compensation plus punitive damages for losses to their industry.

The National Oceanic and Atmospheric Administration on Sunday closed the waters from the Florida Panhandle to the mouth of the Mississippi River to commercial and recreational fishing for 10 days because of the spill.

"We will continue to explore every possible option to create jobs and support local economies in the Gulf while continuing to monitor any potential effects on the national economy," Obama said.

BP drills relief well

Eight robotic submersibles continue to try to activate the well's blowout preventer, a valve on the wellhead that experts believe was badly damaged by a pressure surge that may have caused the Deepwater Horizon rig to explode on April 20 and then sink two days later.

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Welders work near the BP pollution containment chamber, being built in an effort to help contain the massive oil leak, at Wild Well Control Inc. in Port Fourchon, La. ((Sean Gardner/Reuters))

BP, which had leased the Deepwater Horizon and is responsible for cleanup operations, is deploying chemical dispersants both on the water's surface and underwater that turn the oil into clumps that sink to the ocean floor. 

Additionally, BP began drilling a relief well on Sunday "to intercept and isolate" the leaking oil well, the company announced Monday. The aim is to drill down to about 5,486 metres, and inject heavy drilling fluids and then cement to cut off the flow of oil to the surface, chief operating officer Doug Suttles said.

BP is also constructing a collection device — a 67-tonne concrete-and-metal box — that would cover the leaks and channel the oil gushing out of them to a barge on the water's surface. 

Company chief executive officer Tony Hayward said the device, called a cofferdam, will be on the seabed by Thursday and hooked up to a drill ship by the weekend.

He cautioned, however, that there were no guarantees the procedure will work, as it has never been attempted at a depth of 1,500 metres.

Drilling in California affected

While the oil spill threatens states along the Gulf of Mexico, other states are also watching with interest.

In California, Gov. Arnold Schwarzenegger withdrew his support for the Tranquillon Ridge project, a plan for drilling about eight kilometres off the coast of Santa Barbara.

Schwarzenegger had endorsed the $2-billion US endeavour in 2009 in the hopes of easing his state's $20-billion deficit.

On Monday, the governor said the risk of a spill to wildlife is not worth the money.

"I see on TV the birds drenched in oil, the fishermen out of work, the massive oil slick [in the Gulf of Mexico] destroying our precious ecosystem," Schwarzenegger said. "That will not happen in California."

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Workers line up to start the day to help clean up the oil spill, in Venice, La., on Tuesday. ((Alex Brandon/Associated Press))