Nearly 95 per cent of the habitat being designated for polar bears is sea ice in the Beaufort and Chukchi seas. The bears are considered a threatened species. ((U.S. Fish and Wildlife Service))

The U.S. government is setting aside 484,000 square kilometres in Alaska as a "critical habitat" for polar bears, an action that could add restrictions to future offshore drilling for oil and gas.

The total, which includes areas of sea ice off the Alaska coast, is infinitesimally less than in a preliminary plan released last year.

Tom Strickland, assistant secretary of the interior for fish, wildlife and parks, said the designation would help polar bears stave off extinction, recognizing that the greatest threat is the melting of Arctic sea ice caused by climate change.

"This critical habitat designation enables us to work with federal partners to ensure their actions within its boundaries do not harm polar bear populations," Strickland said. "We will continue to work toward comprehensive strategies for the long-term survival of this iconic species."

Designation of critical habitat does not in itself block economic activity or other development, but requires federal officials to consider whether a proposed action would adversely affect the polar bear's habitat and interfere with its recovery.

Nearly 95 per cent of the designated habitat is sea ice in the Beaufort and Chukchi seas off Alaska's northern coast. Polar bears spend most of their lives on frozen ocean and use it to hunt seals, breed and travel.

Alaska Gov. Sean Parnell and the state's oil and gas industry had complained that the preliminary plan released last year was too large and dramatically underestimated the potential economic impact. The designation could result in hundreds of millions of dollars in lost economic activity and tax revenue, they said.

The U.S. Fish and Wildlife Service said reductions included in the final zone were mostly due to corrections that more accurately reflect the U.S. border in the Arctic Ocean.

Aboriginal Alaskan communities in Barrow and Kaktovik, Alaska, are exempt from the restrictions.

The U.S. Interior Department has declared polar bears "threatened," or likely to become endangered, citing a dramatic loss of sea ice.