Polar bears are threatened -- likely to become endangered -- because their sea ice habitat has melted dramatically and computer models predict further losses, Kempthorne said. Polar bears use sea ice for mating, denning and hunting.
Kempthorne said the best scientific judgments did not conclude that polar bears were threatened by oil and gas development.
The conservation groups do not agree.
Whit Sheard, of Pacific Environment, said Bush administration officials have been so keen to grant offshore leases, they have not given proper consideration to the potential harm to polar bears.
"Instead of actively seeking to protect polar bears, they've been aggressively seeking to promote oil and gas development in polar bear habitat," Sheard said.
The groups say the Bush administration has opened up virtually all of Alaska polar bear habitat to leasing.
They are not seeking to shut down offshore drilling, Sheard said, but will sue to ensure that the government follows requirements of the Endangered Species Act.
The act requires federal agencies to ensure that any action they carry out does not jeopardize a listed species, said Brendan Cummings, an attorney for the Center for Biological Diversity. All ongoing federally authorized offshore oil-industry actions affecting the polar bear, from exploration plans to seismic surveys, must be re-examined, Cummings said.
Boats, aircraft and drilling platforms will add to bears' stress by causing them to flee and expend more energy, Cummings said.
"These are animals that, because of global warming, are food-stressed and they're simply in worse physical condition than they would be in an otherwise intact Arctic," he said.
In 1993, the U.S. Fish and Wildlife Service began issuing incidental take permits -- which allow incidental harm of species -- for petroleum exploration. Since then, there has not been a single polar bear or walrus death attributable to oil and gas development, said Bruce Woods, a spokesman for the service.
Alaska has two populations of polar bears. The Chukchi and Bering sea population off the state's northwest coast is shared with Russia. The southern Beaufort Sea population off the state's north coast is shared with Canada.
A 60-day notice of intent to sue is required before a lawsuit can be filed.