A sense of doom enveloped the American coastline from Louisiana to Florida today as the massive BP oil slick spewing from a ruptured well kept growing.

Experts warned that an uncontrolled gusher could create a nightmare scenario if the Gulf Stream carried it towards the Atlantic.

US president Barack Obama will visiting the region today to assess the situation amid growing criticism that the government and oil giant BP should have done more to stave off the disaster.

Meanwhile, efforts to stem the flow and remove oil from the surface by skimming it, burning it or spiking it with chemicals to disperse it continued with little success.

"These people, we've been beaten down, disaster after disaster," said Matt O'Brien of Venice, Louisiana, whose fledgling wholesale shrimp dock business is under threat.

"They've all got a long stare in their eye. They come asking me what I think's going to happen. I ain't got no answers for them. I ain't got no answers for my investors. I ain't got no answers."

As the spill surged toward disastrous proportions, critical questions lingered: who created the conditions that caused the gusher?; did BP and the US government react robustly enough in its early days?; and, most important, how can it be stopped before the damage gets worse?

The US Coast Guard said yesterday that it was nearly impossible to know how much oil has gushed since the April 20 Deepwater Horizon rig explosion, after saying earlier it was at least 1.6 million gallons - equivalent to about two and a half Olympic-sized swimming pools.

The blast killed 11 workers and threatened beaches, fragile marshes and marine mammals, along with fishing grounds that are among the world's most productive.

Even at that rate, the spill should eclipse the 1989 Exxon Valdez incident as the worst US oil disaster in history in a matter of weeks. But a growing number of experts warned that the situation may already be much worse.

The oil slick over the water's surface appeared to triple in size over the past two days, which could indicate an increase in the rate that oil is spewing from the well, according to one analysis of images collected from satellites and reviewed by the University of Miami.

While it is hard to judge the volume of oil by satellite because of depth, it does show an indication of change in growth, experts say.

"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 Centre for Southeastern Tropical Advanced Remote Sensing.

"Clearly, in the last couple of days, there was a big change in the size."

Doug Suttles, BP's chief operating officer for exploration and production, said it was impossible to know just how much oil was gushing from the well, but the company and government officials were preparing for the worst-case scenario.

In an exploration plan and environmental impact analysis filed with the federal government in February 2009, BP said it had the capability to handle a "worst-case scenario" at the Deepwater Horizon site, which the document described as a leak of 162,000 barrels per day from an uncontrolled blowout - 6.8 million gallons each day.

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 famed warm-water current that flows from the Gulf of Mexico to the North Atlantic. Several experts said that if the oil entered the stream, it would flow around the southern tip of Florida and up the eastern seaboard.

The real threat, however, lurked offshore in a swelling, churning slick of dense, rust-coloured oil the size of Puerto Rico.

From the endless salt marshes of Louisiana to the white-sand beaches of Florida, there is uncertainty and frustration over how the crisis got to this point and what will unfold in the coming days, weeks and months.

The concerns are both environmental and economic. The fishing industry is worried that marine life will die - and that no one will want to buy products from contaminated water anyway. Tourism chiefs are worried that holidaymakers will shun oil-tainted beaches and environmentalists are worried about how the oil will affect the countless birds, coral and mammals in and near the Gulf.