The Scottish coastguard mounted a rescue attempt on Friday after receiving reports that a helicopter transporting 18 people between oil rigs in the North Sea had ditched.
The coastguard said lifeboats had been dispatched to the reported crash site near Shetland after the alarm was raised at 7.00pm (1800GMT).
It is not clear whether there are any casualties, according to police in Scotland, who said they were working with the coastguard.
The aircraft is believed to have been travelling from Aberdeen in northeast Scotland to oil and gas platforms when it came down.
It is the latest in a series of incidents involving helicopters in the North Sea.
In May, all 14 people on board a Super Puma helicopter were rescued after it ditched off the coast of Aberdeen.
Another helicopter ditched in the North Sea last October, but all 19 people on board survived.
Sixteen men died when a Super Puma helicopter plunged into the sea when its gearbox failed as it was flying from BP’s Miller platform to Aberdeen in April 2009.
Helicopter carrying 18 ditches in North Sea: reports
