The order, issued Thursday by the 9th U.S. Circuit Court of Appeals, comes after the federal Minerals Management Service in February approved Shell's offshore exploration plan for the Beaufort Sea.

"Vessels currently located in the Beaufort and Chukchi seas shall cease all operations performed in furtherance of that program, but need not depart the area," the order said.

Opponents contend that the Minerals Management Service approved Shell's plan without fully considering that a large spill would harm marine mammals, including bowhead and beluga whales. They say polar bears could also be harmed, and they question whether cleaning up a sizable spill would even be possible in the icy waters.

Company officials are obviously disappointed, said Shell spokesman Curtis Smith.

"But the court has asked for more information, and we will provide it. We will comply with the court order and continue to welcome discussions with the North Slope communities," he said. "Alaska is a long-term investment for Shell."

Shell was the high bidder in two recent lease sales for offshore tracts in the Arctic. In 2005, Shell Exploration & Production Co., part of Royal Dutch Shell PLC, spent more than $44 million for offshore leases in the Beaufort Sea.