The US military may be called in to help contain a massive oil slick that has prompted the southern US state of Louisiana to declare a state of emergency.

The US defence department is on standby after officials said the leak was five times worse than previously thought, and could trigger a disaster of "national significance" on the Gulf coast.

Bobby Jindal, the governer of Louisiana, declared a state of emergency on Thursday when officials warned the slick could reach the shoreline as early as Friday, where its impact would be devastating for the environment.

A massive operation to mitigate the damage is already underway, but needs to be scaled up significantly if it is to be sucessful.

BP, the British energy company responsible for the damage caused by the leak, said it was grateful for the offer of help from the military.

"We'll take help from anyone, I mean we welcome the offer from the department of defence, we're working with the experts across the industry," Doug Suttles, chief operating officer of BP's exploration and production unit, said.

"We're going to do everything we can to minimise the impact of this event." 

The oil leak began on April 20 after an explosion at the Deepwater Horizon drilling rig left 11 workers dead and led to it sinking two days later.

Disaster looms

The spill has sparked fears of an environmental disaster along the US Gulf coast, which could threaten wildlife as well as economically important fishing grounds and oyster beds.

Clean-up crews have been struggling to control the slick, with a fleet of vessels dispatched earlier in the week hampered by strong winds and high seas.

Brandon Blackwell, a US coastguard spokesman in Louisiana, told Al Jazeera that authorities wanted to "fight this spill as far off shore as possible" but were expecting oil to hit the shoreline on Friday.

"Of course it will hurt the environment, local economies will be impacted," he said.

Blackwell advised people to stay away from coastline suffering from the oil spill, but that if people see unattended oil to let authorities know.

At a special White House briefing on the slick, the US coast guard said on Thursday it is being very aggressive in its response to the spill, but admitted at the same time is preparing for the worst case scenario of oil reaching shore.

Coast Guard Rear Admiral Sally Brice O'Hare said that the oil could reach landfall in the Mississipi Delta region by late Friday.

Leaking well

On Wednesday, BP engineers working with the coast guard set fire to parts of the slick, testing a technique to burn off some of the oil and slow its spread. Burning tests were expected to resume on Thursday.

An estimated 5,000 barrels of oil a day are still leaking into the Gulf [AFP]

Neither the US coast guard nor BP offered any new information on efforts to seal off the underwater well head that has caused the massive oil slick.

Four remote-controlled robotic submarines deployed to the leak site earlier in the week have so far failed to activate a shutoff device, called a "blowout preventer", at the head of the well.

As an emergency back-up, BP engineers are also working to construct a giant dome to place over the leaking well to contain it. Collected oil could then be pumped out of the structure.

Prentice Danner, a spokesman for the US coast guard, said that option will take between two to four weeks, and is so far an untested one.

A more sustainable plan to stem the oil flow - drilling a relief well to take the pressure off the initial one - is due to commence on Thursday, but BP has said that effort could take up to three months to complete.