A helicopter with 13 people on board crashed off western Norway on Friday, with rescue workers saying there was no sign of survivors but that the search was going on.Several people could be seen in the ocean, while the helicopter was on a small island, an eyewitness told news channel TV2.TV2 images showed plumes of smoke rising from the site.

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," a local resident told local daily Bergensavisen.

"Pieces (of the helicopter) flew into the air," she said, adding that she saw a rotor blade detach.

Some media outlets reported that people had been rescued, but the reports were contradictory and Bang Andersen was not able to confirm or deny them.

"Horrible reports of a helicopter crash," Prime Minister Erna Solberg tweeted. "I'm being continuously briefed on the rescue operations."

Live footage showed leisure boats rushing toward the scene, where thick black smoke was billowing into the sky.

The accident took place around noon (1000 GMT), and more than an hour later boats could be seen criss-crossing across the water as helicopters hovered overhead. Divers were seen at the site, and ambulances were parked on shore.

On Twitter, police urged people to refrain from using drones in the area.

The helicopter was returning from the Gullfaks B platform, in one of Norway's biggest offshore oil fields, which is operated by state-owned Statoil.

Media reports had initially spoken of up to 17 people on board.