Crude oil oozed into US wetlands Friday as furious Louisiana officials accused BP of destroying fragile marshes and leaving coastal fishing communities in ruin.

As delicate marshlands in the Mississippi Delta faced an environmental nightmare, BP conceded after days of pressure that it had underestimated the amount of oil leaking into the Gulf of Mexico.

Just how much oil is gushing from a pipe ruptured when the Deepwater Horizon rig exploded and sank a month ago has been a contentious issue, with BP putting the figure at 5,000 barrels -- or 210,000 gallons -- a day.

But BP spokesman Mark Proegler said Thursday that 5,000 barrels were being siphoned away each day from the leak by a mile-long (1,600 meter) 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 told AFP.

Independent experts have warned the flow could be at least 10 times higher.

With thick patches of oil tarring coastal Louisiana marshes, a haven for migratory birds and rare wildlife that will be virtually impossible to clean up, local leaders have started to despair.

"Twenty-four miles (nearly 39 kilometers) 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. He said the slick was "destroying our marsh, inch by inch," and would keep on coming ashore for weeks and months.

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 sealing 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," he said after overflying the stricken areas.

"We are very concerned that this is just the beginning," Jindal said, warning it could be too late to save 60,000 jobs in Louisiana's three-billion-dollar fishing industry.

"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."

The neighboring states of Alabama and Mississippi have also been affected by the toxic slick, and Florida's tourist beaches and coral reefs could be next.

The state's governor extended a state of emergency to four southern counties as experts said the slick had entered a powerful current dragging it towards beaches and fragile reefs.

The Loop Current could carry oil ashore within days, as well as 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.

On Thursday, US Interior Secretary Ken Salazar said he was hopeful BP would succeed in 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.

The administration also kept up pressure on British Petroleum, with Homeland Security Secretary Janet Napolitano and Environmental Protection Agency Administrator Lisa Jackson demanding daily public updates on the leak.

"The public and the United States government are entitled to nothing less than complete transparency," they told the oil fim, which on Thursday bowed to Democratic pressure to provide a live online video feed of the leak.