Energy giant BP has been forced to concede it underestimated the amount of oil leaking from the sunken Deepwater Horizon rig, as the slick reached the Louisiana shore.
Environmental disaster fears are being realised in the marshlands of the Mississippi Delta, where crude oil is oozing into fragile wetlands, threatening to damage them beyond repair.
The British firm had always maintained that only 5,000 barrels - or 210,000 gallons - of crude was gushing each day from a pipe ruptured when its Deepwater Horizon rig exploded and sank a month ago.
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But BP spokesman Mark Proegler said on Thursday that this amount was already being siphoned away from the leak by its mile-long insertion tube device and live television pictures showed a significant quantity still streaming out.
"Now that we are collecting 5,000 barrels a day, it might be a little more than that," he said.
Fragile wetlands damaged
Independent experts have warned the flow could be at least 10 times higher.
With thick patches of oil now flooding over coastal Louisiana marshes, a haven for migratory birds and rare wildlife that will be nigh-on impossible to clear up, local leaders were starting to despair.
"Twenty-four miles (38km) of Plaquemines Parish is destroyed. Everything in it is dead," Billy Nungesser, head of the parish in southern Louisiana, told US cable news station MSNBC.
"There is no life in that marsh. You won't clean it up.
"We've been begging BP to step up to the plate," said Nungesser. The slick is "destroying our marsh,h," and will keep on coming ashore for weeks and months, he said.
Work to seal damaged well
An increasingly desperate BP says a "top kill" operation to try to cap the leak for good by filling the well with heavy drilling fluids and then seal it with cement could begin as early as Sunday.
But for Louisiana's fragile wetlands the measure may come too late.
Louisiana Governor Bobby Jindal has been on a personal crusade in recent weeks to force the US government and BP to build sand islands to protect the shoreline and fragile island nature reserves.
"It is clear from what we saw on Fourchon Beach and Thunder Bayou today that the oil is here. It is in our marsh, like we saw yesterday in Pass a Loutre, and it is on our shores," Jindal said, after flying over those stricken areas.
"This oil has travelled 110 miles (177km) to land on our coast and we are very concerned that this is just the beginning," he said, fearing it could be too late to save 60,000 jobs in Louisiana's three-billion-dollar fishing industry.
Seafood industry hard hit
"Louisiana produces nearly one-third of the seafood for the continental US and 70 per cent of the seafood production in the Gulf of Mexico comes from Louisiana fishers, shrimpers and oyster harvesters," he said.
"This is why we have repeatedly said that this spill fundamentally threatens Louisiana's way of life. The oil is here, but we are still waiting on the US Army Corps of Engineers to approve our sand boom plan to help keep oil out of our marshes and off of our shores."
It is not just Louisiana that fears the worst from the giant slick. Neighbouring Alabama and Mississippi have already been affected and Florida's tourist beaches and coral reefs could be next.
The slick is now being swept toward Florida's tourist beaches and fragile coral reefs by the powerful Loop Current, that could wash oil ashore within days, and carry it up the US East Coast and even into the Gulf Stream.
Oil in the current could cause massive damage to the rich marine wildlife that rides the ocean super-highway from spawning zones to areas where they mature, experts warned.
Fears over chemical dispersants
US Interior Secretary Ken Salazar raised expectations about the chances of success for finally sealing the leak from the well.
"Our hope is what they call the dynamic kill of this well will happen on Sunday. And that it will be killed. Everything is being done to make sure that happens," Salazar told CNN.
"Our priority is to stop the well from leaking because that's where the cancer is and we need to get that stopped."
Amid rising concerns that the chemical dispersants used by BP may also be endangering marine life, US officials have given the firm a 24-hour deadline to choose a less toxic chemical to break up the slick.
"This is a large amount of dispersants being used, larger amounts than have ever been used, on a pipe that continues to leak oil and that BP is still trying to cap," an official from the Environmental Protection Agency said.
