Environmental officials say an oil spill in an Alaskan wildlife sanctuary that followed a shipwreck three weeks ago is far worse than originally feared.

Up to 1.28 million litres of thick fuel oil – more than eight times the original estimate – are believed to have leaked into the Bering Sea after a Malaysian-flagged freighter ran aground off the Aleutian islands on Dec. 8.

A spokesperson for the Alaska Department of Environmental Conservation, Lynda Giguere, said Thursday that more than 600 birds have been coated with oil while 109 others have died since the spill was first reported.

Beaches in the area are coated by a thick layer of oil and tar balls have been seen floating in the region's waters, she said.

The Wildlife Refuge is the nesting haven for 40 million seabirds and numerous marine mammals, including the endangered Steller sea lion and western Alaska sea otter.

The Singaporean-owned freighter, Selendang Ayu, was carrying soy beans from the United States to China when it ran aground off Unalaska Island on Dec. 8 after losing power to its engines.

Six crew members died while an American Coast Guard helicopter was trying to airlift them to safety.

With files from the Australian Broadcasting Corporation.