At least eight people have died after a helicopter with 16 people on board crashed into the North Sea.
A major search was launched after the Bond Super Puma came down about 15 miles off Peterhead on the Aberdeenshire coast, at about 1400 BST.
Police later said eight bodies had been recovered and the eight others were still missing.
The crashed helicopter was returning from BP's Miller field, 168 miles north east of Aberdeen.
RAF spokesman Michael Mulford said three helicopters and a Nimrod had been sent to the scene.
He said: "We understand there was a mayday call involving this helicopter and then silence.
"That is an ominous combination."
SUPER PUMA AS 332L
Four-bladed helicopter used primarily by offshore oil firms
Capacity: Up to 25 passengers
Max. speed: 278 km/h (172.7mph)
Range: 776 km (482 miles)
The crash comes less than two months after a Super Puma helicopter with 18 people on board ditched in the Etap field 125 miles east of Aberdeen.
All those on board survived the crash on 18 February.
Helicopter operator Bond confirmed one of its aircraft was involved in the latest incident. It carried 14 passengers and two crew.
Two liferafts were spotted in the water, and were both overturned. All on board would have been wearing survival suits.
Head of fleet operations at the RNLI, Hugh Fogarty, said weather conditions in the area were believed to be good.
He added that the temperature in the North Sea was thought to be around 5C at this time of year.
"People who travel on these helicopters to and from the rigs wear proper immersion suits," he added.
"The survival time, with the proper equipment, could be a number of hours. We are certainly not giving up hope yet."
Scotland's First Minister Alex Salmond described the chances for survivors as "extremely bleak".
"For the families concerned, it's an appalling tragedy and also for everybody who's worried at this moment, about whether their loved ones have been caught up in this tragedy.
"The whole nation expresses its condolences for those who have been tragically killed in this disaster."
Mr Salmond, who represents north east constituencies, cancelled a planned trip to the Scotland's World Cup qualifier against Iceland to go to the Grampian Police headquarters in Aberdeen, who have been co-ordinating the response with the Coastguard search and rescue.
Aviation expert Jim Ferguson on the safety record of the Bond Super Puma'
Prime Minister Gordon Brown said: "This has been a tragic day in the North sea and my thoughts are with the relatives of those who have lost their lives in this crash.
"It is at times like these that we remember the risk and the dangers that people have to undergo working to meet our energy needs.
"My thoughts are with the families and my thoughts are also with those who are doing their best to deal with the consequences of this crash, all those in our hospital services and all those who are on sea and on land trying to help those who have been under difficulty."
Jake Molloy of the OILC union said: "When helicopters hit the water hard it is inevitable you will have fatalities. No amount of training can prepare you for a heavy landing in the sea.
"These aircraft are designed with lightweight equipment and heavy engines on top. They are simply not designed for going into the sea hard."
An NHS Grampian spokeswoman said the major incident plan, which had been launched at Aberdeen Royal Infirmary, had now been stood down.
"We are not expecting anyone in at this moment in time," she said.
"We will reassess that situation if it changes."
An emergency telephone number has been set up by BP for concerned relatives - on
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