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A helicopter has ditched in the North Sea off the coast of Shetland.

The aircraft is reported to have had 18 people on board when it hit the water two miles west of Sumburgh airport at about 6.20pm on Friday.

A NorthLink ferry travelling between Shetland and Aberdeen via Orkney, which set sail from Lerwick at 5.10pm, is said to turned back to assist in the rescue.

The helicopter is a Super Puma AS332 L2 operated by Canadian company CHC, a company that transports offshore workers to and from North Sea platforms.

Police and ambulance crews are standing by on the shore at Lerwick.

Two coastguard helicopters and lifeboat crews from Lerwick and Aith RNLI stations have been sent to the scene.

The BBC reported nine people had been brought on shore by a rescue helicopter, eight of whom were described as walking wounded while the ninth was taken off on a stretcher.

The RNLI said a search was ongoing for three people missing in the water. Some of those on board have been rescued but it is not clear how many.

Conditions in the area are said to be foggy, with high winds earlier in the evening which have since died down.

A spokesman for the operating company said: "CHC Helicopter can confirm that there has been an incident involving one of our aircraft in the North Sea, approximately two miles off Sumburgh.

"Exact details of the incident, which happened at approximately 6.20pm are not yet known.

"The appropriate authorities have been informed and the company’s Incident Management Team is being mobilised."

Investigators from the Air Accidents Investigation Branch (AAIB) are travelling to the scene.

A spokesman for the Department for Transport said: "The AAIB is aware of the incident and has deployed a team".

Four years ago 16 men died when a Super Puma AS332 L2 plunged into the sea. An accident inquiry found it suffered a catastrophic gearbox failure while carrying offshore workers to Aberdeen.

The aircraft, operated by Bond helicopters aircraft, was returning from the BP Miller platform when it went down off the Aberdeenshire coast on April 1, 2009.

More recently a different Eurocopter model, the EC225, was grounded following two ditching incidents in the space of six months last year. Fourteen people were rescued off the coast of Aberdeen in May, while another 19 were rescued after an EC225 experienced difficulty off the coast of Shetland in October.

The EC225 fleet was only cleared to return to service earlier this month after extra safety checks were introduced.

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