"Polar bears are one of nature's ultimate survivors, able to live and thrive in one of the world's harshest environments," he said. "But we are concerned the polar bears' habitat may literally be melting."

An Interior Department official who did not wish to be named told AFP that the proposed listing marks the first time the US government has acknowledged a direct link between global warming and its potential effects on a species.

"We have not had a species that's been listed with such a close correlation to climate change as this one," he said. "This is about as close a correlation between rising temperatures and a species."

The endangered category is reserved for species facing extinction.

There are between 20,000 and 25,000 polar bears worldwide, 4,700 of which live in Alaska and travel to Canada and Russia. Environmental groups for years have raised the alarm about their possible extinction because of global warming.

Warmer temperatures have caused the ice cap to melt, shrinking the bears' hunting grounds and making it increasingly difficult for them to find food.

Identifying polar bears as threatened under the Endangered Species Act could force US industries to reduce their carbon dioxide output to protect the Arctic predators.

Kempthorne underlined, however, that any action to curb greenhouse emissions to protect the bears was beyond the scope of his agency.

He also said that his agency had determined that onshore and offshore oil and gas development in Alaska do not pose a threat to the species.

Wednesday's action was prompted by a petition filed by three environmental groups -- the Center for Biological Diversity, the Natural Resources Defense Council (NRDC) and Greenpeace -- which charged that the US government was not acting quickly enough to protect the bears. The deadline for responding to the petition was Wednesday.

"This is a watershed decision in the way this country deals with climate change," Kassie Siegel, director of the Center for Biological Diversity, told AFP. "The science of global warming and the impact to polar bears are so clear that not even the Bush administration can deny that polar bears are threatened with extinction because of global warming."

Andrew Wetzler, a senior attorney at NRDC, said Wednesday's action was long overdue and would force the government to seriously address the plight of polar bears.

"The time for half-measures and delay is over," he said. "We must face the scientific warnings and address the challenge now."

The US government now will seek public comment on its proposal and decide within a year on whether to officially list the species as threatened, Kempthorne said.

Experts, however, say it is unlikely the proposal will not go through, since the government has already received more than 200,000 comments in support of listing the polar bear.

The United States is the world's biggest emitter of greenhouse gases, the byproduct of fossil fuels blamed for trapping heat from the sun and altering Earth's delicate climate system.

Since taking office in 2001, US President George W. Bush has come under heavy criticism by environmental groups for failing to ratify the Kyoto Protocol on global warming.

Related Links
US Fish and Wildlife Service
Beyond the Ice Age