The US government has listed the polar bear as an endangered species after a three-year battle, but also introduced a caveat that has ignited a fresh row over the scope of the Endangered Species Act.

Secretary of the interior Dirk Kempthorne announced the classification this week, placing the polar bear under the protection of the Endangered Species Act.

This should make it much more difficult for oil companies to explore and drill for oil in large areas of the Arctic, said Kassie Siegel, executive director of the Center for Biological Diversity, who argued that most current Arctic gas and oil production facilities are located onshore.

"The government will have to prepare a recovery plan, and designate critical habitat," she said. "There will be a much higher level of environmental review. "

However, Kempthorne also announced a regulation that would exempt greenhouse gases from consideration as a threat to the polar bear. The regulation, called the interim 4d rule, was introduced "with immediate effect" by the government, but is also open for comment for 60 days. It dismisses greenhouse gases resulting from actions carried out or authorised by the federal government as having an effect on animal habitats.

"Listing the polar bear as threatened can reduce avoidable losses of polar bears," Kempthorne said. "But it should not open the door to use the ESA to regulate greenhouse gas emissions from automobiles, power plants and other sources."

The regulation would negate what environmental lobby groups had hoped would be a new requirement arising from the classification of the polar bear under the ESA. That requirement could have forced the government to consider the climactic effects of any action it authorised, including setting fuel-efficiency standards.

Seigel said the interim rule was illegal. "What they have asserted here contradicts the law. The president is supposed to carry out the law. The president does not have the right to change the wording of the law," she said.

The Centre for Biological Diversity is considering ways to challenge the introduction of the regulation.