A vast oil slick drifting in the Gulf of Mexico has reached the U.S. coastline in Louisiana, imperiling the region's rich sea life and causing what could be one of the country's worst-ever environmental disasters.

As officials grappled with the nearly impossible task of stopping the oil's spread, local fisherman scrambled to reel in their catches before the area was fouled.

David Kennedy of the National Oceanic and Atmospheric Administration said the landfall could be disastrous.

"I am frightened. This is a very, very big thing. And the efforts that are going to be required to do anything about it, especially if it continues on, are just mind-boggling."

Earlier in the day, officials discovered that the slick is much worse than first thought, with as many as 5,000 barrels of crude oil leaking each day -- five times more than previous estimates.

The spill was caused by last week's explosion of the Deepwater Horizon drilling rig, about 200 kilometres southeast of New Orleans.

For reasons still not clear, the rig exploded and caught fire on Apr. 20, then sank two days later. Eleven rig workers went missing in the blast and are presumed dead.

It had been estimated that about 1,000 barrels of sweet crude were being discharged into the gulf. But the U.S. National Oceanic Atmospheric Administration now estimates the well is really spilling about 5,000 barrels a day into the water.

Though crews operating underwater robots tried to activate a cutoff valve on the ocean floor to stop the leak, they failed. The spill has now grown to about 950 kilometres in circumference and is about 75 kilometres wide at its widest point, the U.S. Coast Guard said.

BP is hoping it will be able to cover the well with a steel cap to avert an environmental disaster. But that could take four weeks to put in place, by which time over 150,000 barrels will have spilled.

If the steel cap does not work, BP will then have to drill a relief well, which would take two to three months to complete. By that time, the spill could be over 300,000 barrels -- making it a bigger leak than that caused by the Exxon Valdez disaster, which remains the U.S.'s worst oil spill to date.

Washington has offered to have the Department of Defense use its equipment and expertise to help contain the spill and protect the U.S. coastline.

On Thursday afternoon, BP chief Doug Suttles said at a news conference that the company has asked for military imaging expertise and the use of remote operating vehicles.

"We'll take help from anyone, I mean we welcome the offer from the Department of Defense, we're working with the experts across the industry," Suttles said.

The request came after Louisiana Gov. Bobby Jindal declared a state of emergency as the oil slick moved closer to shore.

Before the oil slick hit Louisiana, some residents said they could already smell it.

"About 30 minutes ago we started smelling it," resident Mitch Jurisich said. "That's when you know it's getting close and it hits you right here."

Some residents spent the day hauling in oyster catches, as it is feared the slick will affect local seafood reserves.

Many residents were shocked by the growing scale of the problem.

Cade Thomas, who works as a fishing guide along the Louisiana coast, said that his livelihood could be destroyed.

He also expressed frustration with the Coast Guard, the federal government and BP.

"They lied to us. They came out and said it was leaking 1,000 barrels when I think they knew it was more. And they weren't proactive," he said. "As soon as it blew up, they should have started wrapping it with booms."

Washington is also springing into action to help the region, which is still recovering from the aftermath of Hurricane Katrina, which tore through in 2005.

President Barack Obama said that the country must use "every single available resource at our disposal" to respond.

Obama has also dispatched Homeland Security Secretary Janet Napolitano, Interior Secretary Ken Salazar and Environmental Protection Agency administrator Lisa Jackson to help.

As owner of the well, BP is financially responsible for the cleanup. The company, which has seen its share price plummet since the disaster began, said the clean-up operation was costing it US$4 million a day. But if the slick reaches land, that estimate will likely increase sharply.

BP and the Coast Guard have already mounted what the London-based company calls the largest oil spill containment operation in history, involving dozens of ships and aircraft.

Workers burned a section of the slick on Wednesday, testing whether it may be used to reduce potential shoreline damage. The company planned to set more fires but it's unclear when those might begin.

A 500-foot boom was also to be used to corral several thousand gallons of the thickest oil on the surface, which will then be towed to a more remote area, set on fire, and allowed to burn.

With reports from Associated Press