A remote-controlled submarine is hunting for a Russian helicopter with eight people on board which went down into the freezing sea off Svalbard, an Arctic archipelago belonging to Norway, rescue officials said.
The Joint Rescue Co-ordination Centres for northern Norway said in a statement that the overnight search "has so far given no results".
Search efforts were being focused near Cap Heer, about 2.5 miles (4km) north of the Barentsburg Heliport, in an area where oil had been spotted on the water and a sonar system mounted on a boat had found an object about 660ft (200m) beneath the surface, local newspaper Svalbardposten reported.
The Mi-8 helicopter went down near the settlement of Barentsburg on Thursday. The aircraft was carrying five crew members and three staff members from Russia's Arctic and Antarctic Research Institute.
It was en route to Barentsburg, Svalbard's second-largest settlement, from the Russian settlement of Pyramiden - a largely abandoned mine that attracts some tourists on excursions to see the empty Soviet-era buildings.
Barentsburg is a Russian coal-mining town of about 500 people. Under a 1920 treaty, Norway has sovereignty over the archipelago but other signatory countries have rights to exploit its natural resources.
Svalbard is more than 500 mles (800km) north of Norway's mainland. It is known for stunning views of snow-covered mountains, fjords and glaciers.
In March 2008, three people were killed when a Russian Mi-8 helicopter with nine people on board crashed near Barentsburg.
In August 1996, a Russian plane carrying residents of Barentsburg and Pyramiden crashed on approach to the Svalbard Airport, killing all 141 people on board.