The Interior Department cites thinning sea ice as the big problem; outside the government, other scientists studying the issue say pollution, overhunting, development and even tourism also may be factors. Greenland and Norway have the most polar bears, while a quarter of them live mainly in Alaska and travel to Canada and Russia.
Interior Secretary Dirk Kempthorne on Wednesday proposed listing polar bears as a "threatened" species on the government list of imperiled species. The "endangered" category is reserved for species more likely to become extinct.
"Polar bears are one of nature's ultimate survivors, able to live and thrive in one of the world's harshest environments," Kempthorne said. "But we are concerned the polar bear's habitat may literally be melting."
A final decision on whether to add the polar bears to the list is a year away, after the government finishes more studies, reports AP.
Such a decision would require all federal agencies to ensure that anything they authorize that might affect polar bears will not jeopardize their survival or the sea ice where they live. That could include oil and gas exploration, commercial shipping or even releases of toxic contaminants or climate-affecting pollution.
Kempthorne, however, said his department's studies indicate that coastal and offshore oil and gas exploration heavily promoted by the Bush administration, particularly in Alaska should not be curtailed.
"It's very clear that the oil and gas activity in that area does not pose a threat to the polar bears," he said.
Similarly, Alaskan natives and other people who depend on hunting the bears as part of their subsistence diet probably will not be affected, Kempthorne said.