Endangered sea turtles and other marine creatures are being corralled into 500 square-mile "burn fields" and burnt alive in operations intended to contain oil from BP's ruptured well in the Gulf of Mexico, the Obama administration confirmed today.

The killing of the turtles – which once teetered on the brink of extinction – has outraged environmentalists and could put BP into even deeper legal jeopardy.

Environmental organisations are demanding that the oil company stop blocking rescue of the turtles, and are pressing the US administration to halt the burning and look at prosecuting BP and its contractors for killing endangered species during the cleanup operation. Harming or killing a sea turtle carries fines of up to $50,000 (£33,000).

"It is criminal and cruel and they need to be held accountable," said Carole Allen, Gulf office director of the Sea Turtle Restoration Project. "There should not be another lighting of a fire of any kind till people have gone in there and looked for turtles."

The Obama administration, confirming the kills, said BP was under orders to avoid the turtles. "My understanding is that protocols include looking for wildlife prior to igniting of oil," a spokeswoman for the National Oceanic and Atmospheric Administration (Noaa) said. "We take these things very seriously."

The agency this week posted a single turtle spotter on the burn vessels, but government scientists are pressing for more wildlife experts to try to rescue the animals before the oil is lit – or at the very least to give them access to the burn fields.

"One can't just ride through an area where they are burning and expect to be safe while looking for turtles. We don't expect that, but we would like to access those areas where we suspect there may be turtles," said Blair Witherington, a sea turtle research scientist at Florida's Fish and Wildlife Research Institute.

More than 425 turtles are known to have died in the spill zone since 30 April, Noaa said.

Conservationists say the losses could imperil the long-term survival of the creatures. All five species of turtles found in the Gulf are endangered or threatened: the Kemp's Ridley most of all.

But in a video posted on YouTube, Mike Ellis, a skipper from Venice, Louisiana, accuses BP of chasing away a boat of conservationists trying to rescue turtles caught in the oil and weed a few miles away from the leak.

"They ran us out of there and then they shut us down," said Ellis.

On days when the weather is fine and there is relatively no wind, BP conducts up to a dozen "controlled burns", torching vast expanses of the ocean surface within a corral of fireproof booms.

Biologists say such burns are deadly for young turtles because oil and sargassum – the seaweed mats that provide nutrients to jellyfish and a range of other creatures – – congregate in the same locations. The sargassum is also a perfect hunting ground for young sea turtles, who are not developed enough to dive to the ocean floor to forage for food.

Once BP moves in, the turtles are doomed. "They drag a boom between two shrimp boats and whatever gets caught between the two boats, they circle it up and catch it on fire. Once the turtles are in there, they can't get out," Ellis said.

The heartbreak for conservationists is that the convergence of sargassum and oil offers the best chance of finding young turtles before they suffocate on the crude. But it can also be deadly.

"When they breathe and come to the surface, they get a mouthful and a bellyful of toxic substance that is very much like wallpaper paste," said John Hewitt, the director of husbandry at the New Orleans aquarium. "If we don't remove them and clean them up, in three or four days that probably spells the end of the turtle."

Since the spill, the aquarium has taken in 90 sea turtles, scrubbing the oil off their shells with toothbrushes and washing-up liquid.

Even before the fires, the two-month gusher in the Gulf of Mexico was threatening the long-term survival of sea turtles.

"This is the worst calamity that I have ever seen for sea turtles," said David Godfrey, executive director of the Sea Turtle Conservancy. "This is really the cradle of sea turtle reproduction for the western hemisphere."The threat to the turtles could continue well after the gusher is capped. The oil spill is turning vast expanses of the Gulf into a dead zone, killing off the jellyfish, crabs and conches that are the staples of an adult diet.

Conservationists are also worried about the survival of the next generation of loggerhead turtles, which are about to climb up on to badly oiled shorelines to begin their nesting season. "They are doomed" said Godfrey.

Godfrey said his organisation was working on plans to dig up about 1,000 nests, or 100,000 eggs, from nesting grounds in the Florida Panhandle and transfer them to hatcheries for safekeeping. "It is a last gasp measure to save 100,000 young sea turtles," he said. "We need every one of these turtles to survive."