Three people are missing after a Super Puma helicopter, carrying 18, crashed into the sea near Shetland.
The Coastguard confirmed that 15 people had been rescued from the sea and were taken to a hospital in Lerwick. An air and sea search is continuing.
The incident happened west of Sumburgh Airport at about 18:20 BST.
The L2 helicopter, carrying 16 people and two crew, was operated by CHC for Total, taking people to and from oil and gas platforms in the North Sea.
Jim Nicolson, RNLI rescue co-ordinator, told the BBC the helicopter was in an "inaccessible" position and that the weather in the area was not "particularly good".
He said: "There was a fresh wind, not overly strong, visibility is not particularly good and it was misty in the area but I doubt if that would have had any impact on causing whatever happened to the helicopter.
"I believe that the helicopter is in a fairly inaccessible position at the moment near the cliffs. There's quite a lot of tide in that area so any person in the water could be carried some distance away.
"It will be becoming much more difficult with darkness but I have no doubt that those involved are putting in every effort to try to obtain the best possible outcome."
One rescue helicopter flew nine people into Lerwick. One was taken off the flight by stretcher. The rest walked off the flight.
A coastguard spokeswoman said passengers had suffered a range of injuries.
She said: "The people that were involved are in varying stages of injury, no-one has walked away from this without a scratch."
A CHC spokesman said: "We can confirm that an L2 aircraft has landed in the water, approximately two miles west of Sumburgh.
"The aircraft was on approach to Sumburgh Airport at approximately 6.20pm when contact was lost with air traffic control.
"We can confirm there were 16 passengers on board, and two crew."
He said the company's Incident Management Team had been mobilised. CHC has set up a helpline for concerned relatives on 01224 296 866.
Michael Bull, whose son Samuel was rescued, said: "We understand he was on his way back from a rig and the helicopter lost power suddenly and immediately ditched into the water.
"He managed to escape straight away because he was right by an exit and I understand soon afterwards that the helicopter turned over."
Police Scotland said a major incident had been declared.
A spokesman said 15 people had been taken to Gilbert Bain Hospital in Lerwick.
Sumburgh Airport has been closed to allow emergency services to deal with the incident.
Airport spokesman Donald Morrison said the helicopter had been making an approach at 18:15 BST and lost radar contact with air traffic control.
Three helicopters and two lifeboats are involved in the search.
The coastguard said the helicopter's life rafts were found empty and wreckage from the helicopter was starting to wash up at Garth Sneff near Sumburgh Head.
A spokeswoman said: "There is still an ongoing search and rescue mission for the three missing people."
Sq Ldr Dave Webster said RAF Kinloss had received a call at 18:27 BST, saying a Super Puma had ditched to the west of Sumburgh - about three or four miles offshore.
A ferry travelling between Shetland and Aberdeen was diverted to the scene and the RNLI launched Aith and Lerwick lifeboats.
Northlink Ferries confirmed that one of its ferries, which had left Shetland and was heading to Aberdeen, had been diverted back to the incident.
A spokeswoman said: "I can confirm that the Hjatland passenger ferry and Helliar freight vessel are providing support.
"We believe there are 201 passengers on board the Hjatland, which was on its way to Orkney to pick up more passengers."
The helicopter was flying from the Borgsten Dolphin oil platform in the North Sea to Sumburgh Airport.
A specialist medical team has been flown from Aberdeen Royal Infirmary to Lerwick by the RAF helicopter from Lossiemouth.
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".
The Unite union's Scottish secretary Pat Rafferty said: "Our immediate thoughts are with those people and their families and we can only hope for good news although as time goes on the situation becomes more worrying."
He added: "Fifteen people have thankfully been rescued and accounted for, unfortunately some with injuries, and their rescue is testimony to the bravery and skills of the rescue services.
"This brings into sharp focus once again the very precarious nature of the transportation of workers to and from offshore platforms. The health and safety of working people is our priority and we will be watching events closely as they happen."
Tavish Scott, the MSP for Shetland, said: "The first thought has to be with those who are unaccounted for. One can only think of families and loved ones who must be desperately worried at this time."
The Moderator of the General Assembly of the Church of Scotland, the Right Reverend Lorna Hood, said: "Our thoughts and prayers are with all those involved in the Super Puma helicopter crash off Shetland and especially those waiting news of their loved ones."
Last year, two helicopters ditched in the North Sea only six months apart. All passengers and crew were rescued in both incidents which were found to be caused by gearbox problems.
In October, 17 passengers and two crew were rescued from life rafts by a passing vessel after the helicopter, which was carrying an oil crew from Aberdeen to a rig 86 miles north-west of Shetland, was forced to ditch.
Previously, in May 2012 all 14 passengers and crew members on a Super Puma helicopter were rescued after it ditched about 30 miles off the coast of Aberdeen.
The helicopter was on a scheduled flight from Aberdeen airport to a platform in the North Sea at the time.
Super Puma EC 225s were grounded in the wake of the two incidents but were given approval to fly again and services resumed earlier this month.
In April 2009, 16 people died when a Super Puma plunged into the sea. Its gearbox failed while carrying the men to Aberdeen.
The Bond-operated aircraft was returning from the BP Miller platform when it went down off the Aberdeenshire coast.