Environmental groups including the National Audubon Society, Natural Resources Defense Council and Earthjustice filed suit in a federal court along with Alaska native groups to stop the lease sale -- which the federal government has put on a fast track for action.

The Chukchi Sea is one of the few "frontier areas" where new oil and natural gas deposits can be found in North America, and could hold 15 billion barrels of oil, according to the Minerals Management Service, which oversees oil and gas leasing for the Interior Department.

Plaintiffs in the suit claim drilling will endanger polar bears, along with bowhead whales, gray whales, Pacific walrus, ribbon seals, threatened spectacled eiders, and other marine birds and fish.

"The only thing keeping pace with the drastic melting of the Arctic sea ice is the breakneck speed with which the Department of Interior is rushing to sell off polar bear habitat for fossil fuel development," said Brendan Cummings, oceans program director at the Center for Biological Diversity, one of the plaintiffs.

A spokesman for the Minerals Management Service declined to comment.

A key decision on whether to list the big Arctic bear as "threatened" under the Endangered Species Act is due in coming weeks from the US Fish and Wildlife Service, which could coincide with the lease sale.

Earlier this month, MMS director Randall Luthi told a congressional panel that the risk to the bears from oil drilling would be negligible.

If the oil sales went through before a decision was reached on the polar bears, there would be "an additional layer of consultation" with conservation officials as oil and gas companies worked in the area, Luthi said.

World polar bear populations are currently stable, but US scientists estimate that two-thirds of them could be gone by 2050 if predictions about melting sea ice hold true. Polar bears live and hunt on sea ice; when it melts, they either drown or are forced onto land, where they are inefficient hunters.