OSLO - Norwegian oil company Statoil ASA said Monday that its U.S. subsidiary has acquired a 25 percent share in 50 ConocoPhillips oil leases in the Chukchi Sea off northwestern Alaska in exchange for an undisclosed sum and shares in Statoil's Gulf of Mexico operations.

The move, which makes Statoil the fourth largest offshore presence in Alaska, is intended to improve the Norwegian company's experience in the area, Statoil spokeswoman Mari Dotterud said. The region is of growing interest to oil companies as retreating Arctic ice makes it easier to drill.

Statoil already owns 16 leases in the Chukchi Sea, a body of water on the edge of the Arctic Ocean, north of the Bering Strait.

"The Chukchi Sea in Alaska is a new frontier area for all operators ... We have now acquired a sizable acreage portfolio to explore in the coming years," said Tony Dore, chief of Statoil exploration in North America.

Statoil shares slipped 1 percent, to 136 kroner ($23.47), in afternoon trading in Oslo. ConocoPhillips stock, meanwhile, rose a quarter of a percentage point to $50.85 on the New York Stock Exchange.

ConocoPhillips, which will operate the leases alone, is set to start drilling in the Chuchki Sea in 2012.

While Statoil won't be involved in operations, it will contribute expertise in drilling in "harsh environments," which it has developed over three decades in the North Sea, Dotterud said.

In addition to an undisclosed amount of money, ConocoPhillips has received 50 percent interest in 16 Statoil-operated leases in the Gulf of Mexico and reabsorbed Statoil's 25 percent holdings in five ConocoPhillips Gulf blocks, Statoil said.

Statoil is the world's largest offshore operator, with 30,000 employees in 40 countries. ConocoPhillips is the third largest oil company in the U.S.