Thick sheets of crude oil spread through the delicate wetlands of Louisiana today, as the BP oil spill continued to threaten the American coastline.

Local reports described heavy sheets of oil the consistency of latex paint clogging the marshes in the Mississippi delta that provide a haven for migratory birds, and buffer the shore from Gulf hurricanes.

"This is what everyone wanted to avoid, because the wetlands are the nursery for everything that swims or crawls in the Gulf of Mexico," said John Hocevar, oceans campaigner for Greenpeace. "Once the oil gets stuck in there we are pretty much stuck with it."

The sightings of heavy crude over the last 24 hours were seen as a far more serious threat to nesting birds, spawning fish and endangered sea turtles than the scattered tar balls and light sheen spotted earlier on the shoreline. "Twenty-four miles of Plaquemines parish is destroyed. Everything in it is dead," Billy Nungesser, head of the parish in southern Louisiana, told MSNBC after a tour of the marshes.

Anger has grown at BP and the government for failing to anticipate and contain the disaster, a month after the sinking of the Deepwater Horizon. Louisiana's Republican governor, Bobby Jindal, has warned for days that the thousands of miles of boom deployed by BP were too flimsy to keep back the oil during bad weather, and that the government needed to build sand barriers.

Florida could be next, with oil now caught up in the powerful Loop Current. Alabama and Mississippi have also been affected by the spill.

In Washington, members of Congress have accused BP and the administration of covering up the scale of the disaster threatening the shoreline as well as the deep waters of the Gulf.