"Right now, pregnant females foraging offshore in summer must wait up to a month longer than they did even 10 years ago for new sea ice to form so they can travel to denning areas on land," USGS researcher Steve Amstrup said in a statement.

"Alternatively, they must swim ever greater expanses of open water to reach suitable land denning habitat or they must den on ice that may not be stable enough to survive the winter."

Eventually, the researchers predicted, the ice will freeze so late that bears will be stranded at sea too far away to reach land safely. Pregnant polar bears must create dens to protect new cubs from the Arctic winter.

Arctic seas are warming and the ice is melting as global temperatures rise.

Amstrup's team used satellite telemetry to see where the big white bears were building their dens.

They said the edge of the sea ice remains as far as 125 miles (200 km) offshore in late September and early October. Only a decade ago, the water was frozen almost to the shore by that time, they reported in the journal Polar Biology.

The researchers said they would make their report available to the US Fish and Wildlife Service as it considers the proposed listing of the polar bear as a threatened species under the Endangered Species Act.

"In recent years Arctic pack ice has formed progressively later, melted earlier and lost much of its older and thicker multi-year component," another researcher, Anthony Fischbach, said.

"Together, these changes have resulted in pack ice that is a less stable platform on which to give birth and raise new cubs."