The US has proposed listing polar bears as a threatened species because of declining Arctic ice levels.
There are about 20,000-25,000 polar bears worldwide
It is the first time the US has made a direct link between global warming and the threat to a species.
President George W Bush has steadfastly refused to back mandatory controls of emissions of carbon dioxide - believed the main gas behind global warming.
There are 20,000 to 25,000 polar bears across the globe, about 4,700 of them in the US state of Alaska.
Oil and gas
US Secretary of the Interior Dirk Kempthorne said polar bears were "one of nature's ultimate survivors".
But he added: "We are concerned the polar bears' habitat may literally be melting."
Being listed as "threatened" is a rung down from being "endangered".
A department official told the AFP news agency the US had not had a species that had been "listed with such a close correlation to climate change as this one".
Mr Kempthorne sidestepped questions about US reductions of gas emissions, saying it was not a question for his department.
But he stressed offshore oil and gas developments in Alaska were not part of the risk.
Lobby groups welcomed the proposal. Kassie Siegel, of the Centre for Biological Diversity, said it was "a watershed decision in the way this country deals with climate change".
However, listing the bears as threatened is still a year away, pending further studies.
The listing would require all federal agencies not to take decisions that would threaten polar bears' survival.
The Swiss-based Polar Bear Specialist Group projects a 30% decline in numbers over the next 45 years.