Rick Steiner, a marine conservation specialist and co-director of environmental NGO "The Coastal Coalition," sat in an airplane looking down at the chocolate brown and orange gunk covering the vast swath of ocean below.

"The extent of oil out there is enormous, as far as you can see to the horizon both to the west and to the east," he said after his observation flight on Friday over the site of the BP oil spill off Louisiana's coast.

"There's so much oil out there, and these guys are going to be dealing with this for months to come," he said, calculating that the crude covered around 500 square miles (804.7 square km) of ocean surface.

And that is just the tip of the oil-slicked iceberg, as the spill could cause extreme damage to ecosystems, marshes and wildlife, experts said.

The ruptured oil well has gushed an estimated 500,000 to one million gallons daily for several weeks. And while BP on Friday said it was siphoning the oil to a ship above in the latest effort to contain the leak, success remains uncertain.

Scientists and officials said the spill could be the worst environmental disaster in U.S. history and exceed the damage caused by the 1989 Exxon Valdez accident that leaked an estimated 10.8 million U.S. gallons on to the shores of Alaska.

Oil has reached the shores of Alabama and Mississippi and scientists said it was moving in the direction of Florida. Some predict currents could take it far up the east coast.

"The problem is that (much of the damage) is out of sight, out of mind to the people on shore, who are mostly concerned about the beaches," Steiner said.

Oil has already started creeping into marshlands -- habitats for many birds that are also important to the area's most commercially valuable fisheries -- and it is seeping into roots and killing reeds. While the area marks only a small portion of the surrounding vegetation, the oil is difficult to clean up, Steiner says.

Birds can die from ingesting the toxic liquid while cleaning it from their feathers and can go blind if oil gets into their eyes, as was the fate of many seals after the 1989 Exxon Valdez spill. Oil in birds' wings also prevents them from flying.

Six hundred animal species are at risk, according to the Louisiana Department of Wildlife and Fisheries, including more than 400 fish species and 100 kinds of birds.

Steiner said, offshore, the pelagic ecosystem was in danger because its host of fish eggs and larvae in their most vulnerable life stages were sensitive to contamination.

The Gulf of Mexico's deepwater coral reef and the "cold seeps" -- where the sea bed emits natural methane and gas -- had spawned many organisms that were also in danger, although some might prove more resilient to the oil because they lived on hydrocarbons, he said.