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The massive oil spill in the Gulf of Mexico could be even worse than thought, as new evidence suggests a blown-out underwater wellhead may be leaking faster than previously estimated, experts conceded Saturday. 

The spill, the worst U.S. oil leak in decades, reached into marshlands along the southeastern coast of Louisiana on Saturday and threatened three other states.

A giant slick of petroleum, 200 kilometres long and 112 kilometres wide, threatens hundreds of species of wildlife, including birds, dolphins, shrimp, oysters and crabs.

The oil has been leaking since last week, when the Deepwater Horizon offshore drilling platform exploded and then collapsed into the Gulf of Mexico about 80 kilometres from the Louisiana coast, rupturing a wellhead 1,500 metres below the ocean's surface.

Initial estimates pegged the leak rate at 1,000 barrels a day, subsequently hiked to 5,000 barrels a day. But on Saturday, the commander of the U.S. Coast Guard, Admiral Thad Allen, said no one could really pinpoint how much oil is leaking from the well because of its depth.

"Any exact estimation of what's flowing out of those pipes down there is probably impossible at this time due to the depth of the water and our ability to try and assess that from remotely operated vehicles and video," Allen said during a conference call.

Doug Suttles, BP's chief operating officer for exploration and production, agreed, saying it was impossible to know just how much oil was gushing.

Worrisome new images

Most disconcerting was new evidence that the oil slick over the water's surface appeared to triple in size over the past two days, indicating a possible increase in the rate that oil is spewing from the well.

An analysis of images collected from satellites and reviewed by the University of Miami found that while it's hard to judge the volume of leaking oil by satellite because of depth, it does show an indication of change in growth, an expert said.

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The spill threatens hundreds of species of wildlife, including birds, dolphins and the fish, shrimp, oysters and crabs that make the Gulf Coast one of the most abundant sources of seafood in the United States.

"The spill and the spreading is getting so much faster and expanding much quicker than they estimated," said Hans Graber, executive director of the university's Center for Southeastern Tropical Advanced Remote Sensing. "Clearly, in the last couple of days, there was a big change in the size."

Oil industry experts and officials are reluctant to describe what, exactly, a worst-case scenario would look like — but if the oil gets into the Gulf Stream and carries it to the beaches of Florida, it stands to be an environmental and economic disaster of epic proportions.

The Deepwater Horizon well is at the end of one branch of the Gulf Stream, the warm-water current that flows from the Gulf of Mexico to the North Atlantic. Several experts said that if the oil enters the stream, it would flow around the southern tip of Florida and up the eastern seaboard.

"It will be on the east coast of Florida in almost no time," Graber said. "I don't think we can prevent that. It's more of a question of when rather than if."

The oil spill is already threatening the Gulf Coast seafood industry, one of the largest in the world.

Casi Callaway, executive director of the environmental group Mobile Baykeeper, told CNN that oil is covering shrimp and oyster beds, where the shellfish are only now hatching after a cold winter.

"Sixty-nine per cent of the oysters and 70 per cent of the shrimp, domestically generated, come from Apalachicola Bay in Florida to Louisiana," she said.

With the wind blowing from the south, the mess could reach the coasts of Mississippi, Alabama and Florida by Monday.

National Guard called out

U.S. President Barack Obama will head to the Gulf Coast on Sunday morning to get an update on efforts to contain the massive spill, a White House official said Saturday.

On Friday, Obama announced the deployment of 6,000 National Guard troops to Louisiana to help clean up and contain the spill.

The U.S. Coast Guard estimates at least six million litres of oil have leaked into the Gulf of Mexico since the Deepwater Horizon, leased by British petroleum giant BP, exploded and collapsed, killing 11 workers.

Ian MacDonald, a professor of oceanography at Florida State University who specializes in tracking ocean oil seeps from satellite imagery, said there may already be more than 34 million litres of oil floating in the Gulf.

The Exxon Valdez oil tanker spilled 42 million litres off Alaska's shores in 1989.

Capping efforts fail

Underwater robots have so far failed to cap the destroyed rig's well on the Gulf seabed.

The weather has also made it impossible for crews to skim oil from the water's surface or burn it off, said Rear Admiral Sally Brice-O'Hara, the deputy commandant of operations for the U.S. Coast Guard.

Meanwhile, documents show BP downplayed the possibility of a catastrophic accident in the Gulf of Mexico.

In its 2009 exploration plan and environmental impact analysis for the well, BP suggested it was unlikely or virtually impossible for an accident to occur that would lead to a giant crude oil spill and serious damage to beaches, fish and mammals.

With files from The Associated Press