The Super Puma chopper went down around midday in the archipelago off the coast of Bergen, Norway’s second-biggest city.
Eleven bodies have been recovered and the two remaining people are presumed dead, rescue services said.
“We presume that all 13 are dead,” Sola rescue centre spokesman Borge Galta told AFP.
Search and rescue operations were called off late Friday afternoon.
The aircraft was carrying 11 Norwegians, one Briton and one Italian, rescue services said.
The cause of the accident was not immediately known.
The helicopter broke into pieces near a small island and debris was found scattered on land and at sea. Part of the chopper was resting on the seabed under five to seven metres (16 to 23 feet) of water, around 20 metres from land, rescue officials said.
Another Sola rescue centre spokesman, Anders Bang Andersen, told AFP the chopper had been on its way to Bergen’s airport when it crashed with 11 passengers and two crew members on board.
It was returning from the Gullfaks B platform, in one of Norway’s biggest offshore oil fields, which is operated by state-owned Statoil.
– ‘Pieces flew into the air’ –
Several witnesses described seeing the aircraft spiral downwards, followed by a powerful explosion, and people were seen in the sea.
“There was an explosion and a very peculiar engine sound, so I looked out the window. I saw the helicopter falling quickly into the sea. Then I saw a big explosion,” an island resident told local daily Bergensavisen.
“Pieces (of the helicopter) flew into the air,” she said, adding that she saw the rotor detach.
The crash was the deadliest helicopter accident in Norway since 1978, when a chopper plunged into the sea, killing 18 people.
“Horrible reports of a helicopter crash,” Prime Minister Erna Solberg tweeted.
Live footage shortly after the crash showed leisure boats rushing toward the scene, where thick black smoke was billowing into the sky.
More than an hour after the crash boats could be seen criss-crossing the water as helicopters hovered overhead. Divers were seen at the site, and ambulances were parked on the shore.
The chopper was an EC225 Super Puma built by Airbus Helicopters and operated by CHC Helikopterservice for Statoil.
Norway said it had grounded all helicopters of the same model, while Statoil had set up an emergency help centre in Bergen for families of the victims.
On August 23, 2013, a Super Puma AS332 L2, an older model of the same helicopter, crashed into the North Sea near the Shetland Islands, killing four.
Source: Oslo (AFP)