An oil spill that threatens to become America's worst environmental disaster in decades, spread out of control and started washing ashore along the Gulf Coast on Thursday night as fishermen rushed to scoop up shrimp and crews spread floating barriers around marshes.

The spill was much bigger than imagined, five times more than first estimated, and closer.

Fingers of oily sheen were reaching the Mississippi River delta, lapping the Louisiana shoreline in long, thin lines.

The oil slick is threatening hundreds of species of fish, birds and other wildlife along the Gulf Coast, one of the world's richest seafood grounds, teeming with shrimp, oysters and other marine life.

"It is of grave concern," David Kennedy of the National Oceanic and Atmospheric Administration said: "I am frightened. This is a very, very big thing. And the efforts that are going to be required to do anything about it, especially if it continues on, are just mind-boggling."

Oil is thickening in waters south and east of the Mississippi delta about five miles offshore.