LONDON (REUTERS) - Three people were missing after a helicopter carrying 18 people crashed into the sea off the coast of Scotland on Friday.

The Coastguard said 15 people had been rescued and were taken to hospital, but three are unaccounted for.

The incident happened near the Shetland Islands, north-east of Edinburgh, and involved a Super Puma helicopter taking 16 passengers and two crew members to and from oil and gas platforms.

The Department of Transport issued an statement on behalf on the Air Accidents Investigation Branch stating it was "aware of incident" and has deployed a team.

An air and sea search is continuing, with three helicopters and two lifeboats involved.

"Our two lifeboats are searching for those three unaccounted for," said Mr Tim Ash, a spokesman for Royal National Lifeboat Institution.

Wreckage from the crash has been spotted, said Mr Ash, who added that rescue teams were dealing with strong tides and poor visibility.

"Winds are not particularly strong but visibility is not good. Those are the circumstances that our volunteers are facing," he said.

A spokesman for the Coastguard said two life rafts from the helicopter were located and found empty.

"They are still searching but they will stand down at some point and then resume at first light," she added.