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."
This is a victory for the polar bear, and all wildlife threatened by global warming
Kassie Siegel,
Centre for Biological Diversity
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.
A family of polar bears. The United States is giving some thought to adding polar bears to the list of threatened species. The proposal to list polar bears has resulted from a lawsuit settlement the Bush administration reached in February with Greenpeace and the Natural Resources Defense Council, and a decsion was to be made 12 months after that settlement, based on the Endangered Species Act petition process.

"Today the Interior Department's U.S. Fish and Wildlife Service is proposing to list polar bears as a threatened species under the Endangered Species Act. We are making this proposal because a scientific review of the species by the Fish and Wildlife service found that populations may be threatened by receding sea ice, which polar bears use as a platform for many activities essential to their life cycle, including hunting for their main prey, Arctic seals," said Dirk Kempthorne, the Secretary of the Interior in the U.S.

"Polar bears are one of nature's ultimate survivors, but we are concerned the polar bears' habitat may literally be melting," added Kempthorne.

"We have never had a species listed with such a close correlation to climate change as this one," said an attorney with Natural Resources Defense Council, Andrew Wetzler.

"Global warming is the single biggest threat to polar bears' survival, and this will require the government to address the impacts on the polar bear," added Wetzler.
right
The "threatened" category encompasses "vulnerable," "endangered," and "critically endangered." Currently, polar bears are listed as "vulnerable" under the World Conservation Union.

Although the United States Fish & Wildlife Service makes no formal mention of increasing or decreasing polar bear population within US borders or worldwide, it is estimated that there are between 22,000 and 25,000 polar bears living, some 4,700 are living in the state of Alaska, and about 15,000 live in Canada.