The US administration has banned oil drilling in new areas off the US coast pending investigations into the cause of the oil spill off Louisiana.

The spill was triggered by an explosion last week off the Louisiana coast that sank an oil rig operated by BP. Eleven workers are missing and presumed dead.

The incident is likely to have repercussions on the already stalled Senate cliimate and energy bill, which as part of several concessions offered for Republican support, includes offshore drilling. The White House and democratic leaders had hoped the bill would be debated before the end of the year, but this is looking politically and environmentally more dim.

The oil slick was only three miles offshore on Thursday afternoon and was expected to hit coastal Louisiana as early as Thursday night, prompting Louisiana Governor Bobby Jindal to declare a state of emergency and to request the participation of the National Guard in response efforts.

About 40,000 feet of boom had been placed around Pass-a-Loutre, the area of the Mississippi River Delta where the oil was expected to touch first, a spokesman for Jindal told reporters.

The Navy provided 50 contractors, seven skimming systems and 66,000 feet of inflatable containment boom, a spokesman said. About 210,000 feet of boom had been laid down to protect the shoreline in several places along the Gulf Coast, though experts said that marshlands presented a far more daunting cleaning challenge than sandy beaches.

The response effort has been driven by BP, the company that was leasing the rig and is responsible for the cleanup, under the oversight of the Coast Guard and in consultation with the Minerals Management Service and the National Oceanic and Atmospheric Administration. While additional federal resources, including naval support, were available before Wednesday, officials had given little indication that such reinforcements would be deployed so quickly and at such a scale.

Exxon Valdez take two?

So far the leak from a blown-out well 5,000 feet under the sea is not nearly as big as the Exxon Valdez disaster, which spilled about 11 million gallons of oil into Alaska’s Prince William Sound 21 years ago. BP’s well is spewing about 210,000 gallons of oil a day into the ocean, the Coast Guard estimates.

“No additional drilling has been authorised and none will until we find out what happened,” White House adviser David Axelrod told ABC television. Up to 5,000 barrels of oil a day are thought to be spilling into the water after last week’s rig explosion.

US President Barack Obama announced he would eased a moratorium on new offshore drilling late last month.

“As it is now, it’s already looking like this could be the worst oil spill since the Valdez,” John Hocevar, oceans campaign director for Greenpeace USA, told msnbc.com on Thursday.

“It’s quite possible this will end up being worse than the Valdez in terms of environmental impact since it seems like BP will be unable to cap the spill for months. In terms of total quantity of oil released, it seems this will probably fall short of Exxon Valdez. But because of the habitat, the environmental impact will be worse.”

The slick has begun to reach the Louisiana shore, and the US Navy has been sent to help avert an economic and environmental disaster.

“Probably the only thing comparable to this is the Kuwait fires [following the Gulf War in 1991],” Mike Miller, head of Canadian oil well fire-fighting company Safety Boss, told the BBC World Service.

“The Exxon Valdez is going to pale in comparison to this as it goes on.”

State of emergency

Axelrod announced the ban on drilling in new areas in an interview with ABC’s Good Morning America programme on Friday.

He also defended the administration’s response to the 20 April explosion that destroyed the BP-operated Deepwater Horizon rig, saying “we had the Coast Guard in almost immediately”.

The US government has designated the Gulf of Mexico oil spill as an “incident of national significance.” This allows it to draw on resources from across the country.

Calling it “a spill of national significance” which could threaten coastline in several states, Homeland Security Secretary Janet Napolitano announced the creation of a second command post in Mobile, Alabama, in addition to the one in Louisiana, to manage potential coastal impact in Alabama, Mississippi and Florida.

The wetlands off the coast of Louisiana sustain hundreds of wildlife species and a big seafood and fishing industry. The states of Mississippi, Alabama and Florida are also threatened, as oil continues to escape from the wreckage of the rig.

Potential environmental catastrophes loom

“If we lose the integrity of that wellhead, it could be a catastrophic spill,” Adm. Thad Allen, commandant of the Coast Guard, which is directing efforts to contain the spreading spill, told The Miami Herald’s editorial board Wednesday.

Greenpeace’s Hocevar said he’s particularly concerned about the impact to critically endangered bluefin tuna. “It’s their spawning season and bluefin larvae in this part of their life-cycle would be near the surface of water,” Hocevar said.

The oil could also harm sea turtles, which are approaching nesting season; fin whales; menhaden, a fish species harvested mostly for fish meal and fish oil; bottom-feeding oysters; and numerous species of birds, Hocevar said.

Experts said the spill could also destroy the livelihood of commercial fishermen and shrimp catchers and impact recreational fishermen. According to the Louisiana Department of Wildlife and Fisheries, the state’s fishing industry is worth $265 billion at dockside and has a total economic impact of $2.3 trillion.

Tourism also could take a blow if beaches are fouled.

Already, a federal class-action lawsuit has been filed on behalf of two commercial shrimpers from Louisiana seeking at least $5 million in compensatory damages plus an unspecified amount of punitive damages against Transocean, BP and other companies linked to the rig blast.