Several people could be seen in the ocean, while the helicopter was on a small island, an eyewitness told news channel TV2, according to Reuters.
TV2 images showed plumes of smoke rising from the site.
"Helicopter crash: Emergency services at the scene. Extent of damage unknown," the regional fire brigade said on Twitter.
Quoting local police, newspaper Verdens Gang said the helicopter had 13 people on board.
The area just west of Bergen, Norway's second largest city, sees frequent helicopter traffic to and from offshore oil installations.