Alabama Governor Bob Riley declared a state of emergency on Friday, saying the oil spill in the Gulf of Mexico posed "a serious threat to our environment and economy."    

His declaration followed similar actions taken by the governors of Louisiana and Florida.

Meanwhile, there is little hope that near-term efforts by London-based BP to choke off a leaking underwater oil well will succeed, experts said, raising the prospect of an environmental disaster on the scale of the 1989 Exxon Valdez oil spill.

BP, the majority owner of an undersea well leaking 5,000 barrels (210,000 gallons or 955,000 litres) a day of oil into the Gulf of Mexico, is hoping two relatively quick fixes bear fruit before a pair of relief wells that could take up to three months to drill allow the company to plug the leak.

In the interim, the coastal states of Louisiana, Mississippi, Alabama and Florida could take a battering as oil flows unhindered.

"At 5,000 barrels a day, in two months' time it's going to be a bigger spill than the Exxon Valdez," said Tyler Priest, director of global studies at the University of Houston's Bauer College of Business. "You're looking at a huge disaster."    

At that rate, it will take about 50 days for the spill to eclipse the 1989 Exxon Valdez spill in Alaska, the worst US oil spill on record, which sent 10.8 million gallons (49 million litres) of crude oil into Prince William Sound.

Underwater robots have so far failed to activate a cutoff valve at the seabed 5,000 feet (1,524 metres) below the ocean surface to stop the leak left when a blowout preventer failed.

BP has ordered three "containment chambers," or giant 73-ton box-shaped inverted funnels, to cover the well and two other leaks and channel the oil to a drillship. But it will take two to four weeks to fabricate the piping necessary to connect the funnel to the vessel.

At least 1.6 million gallons (6 million litres) of oil have spilled so far since the April 20 explosion that killed 11 workers, according to Coast Guard estimates.

One expert said Friday that the volume of oil leaking from the well nearly 5,000 feet (1,500 meters) below the surface could actually be much higher, and that even more may escape if the drilling equipment continues to erode.

"The sort of occurrence that we've seen on the Deepwater Horizon is clearly unprecedented," BP spokesman David Nicholas said. "It's something that we have not experienced before ... a blowout at this depth."

Amid increased fingerpointing Friday, efforts sputtered to hold back the giant oil spill seeping into Louisiana's rich fishing grounds and nesting areas, while the government desperately cast about for new ideas for dealing with the growing environmental crisis.
President Barack Obama halted any new offshore drilling projects unless rigs have new safeguards to prevent another disaster.

The spill - a slick more than 130 miles (200 km) long and 70 miles (110 km) wide -- threatens hundreds of species of wildlife, including birds, dolphins and the fish, shrimp, oysters and crabs that make the Gulf Coast one of the nation's most abundant sources of seafood.

Although the cause of the explosion was under investigation, many of the more than two dozen lawsuits filed in the wake of the explosion claim it was caused when workers for oil services contractor Halliburton improperly capped the well - a process known as cementing. Halliburton denied it.