A Norwegian cargo ship sank off Sweden's southwestern coast and its six crew members were missing, rescue officials said Friday.

The Swedish Maritime Administration said it received distress calls from the Langeland early Friday, and contact was lost shortly afterward. Strong winds were apparently causing high seas at the time.

A helicopter, plane and several rescue boats were sent to the area where the ship is believed to have gone down, in the Koster fjord near the Norwegian border.

Rescuers spotted life jackets, a raft and other debris floating on the water, but there was no signs of the crew members, most of whom are believed to be Russian, rescue spokesman Birger Knutsson said.

"The ship has gone down, but there is still hope to find survivors," he said, adding that the cause of the sinking was unclear.

The Langeland is 230 feet (70 meters) long and weighs 70 tons. It was on its way to Moss in southern Norway, carrying insulation material, Moss Port Manager Reidar Magnus Hansen said.