A total of five people are missing, feared trapped inside a capsized oil rig support vessel in the North Sea.
Rescue teams said 10 of the Norwegian crew had been recovered following the incident at 1700 BST, about 75 miles west of the Shetland coast.
Eight of the 10 were said to be safe but the situation regarding the other two crew members is not yet known.
Six navy divers are being flown to the boat from Faslane on the Clyde in an attempt to locate the trapped five.
Four of the rescued crew were flown to Shetland for treatment.
The Bourbon Dolphin, a new vessel, had been working in the vicinity of the Rosebank oilfield with a crew of 15.
RAF spokesman Michael Mulford told BBC News 24 it would be a "severe test" for the divers.
"Five are believed to be still inside the upturned hull," he said.
"If there is any possibility that these people are in there and alive this will be their one chance of getting out."
Mr Mulford said eight of the rescued men "were known to be safe" but he "did not know for sure" about the other two.
Earlier Mr Mulford told BBC Scotland: "What we know is that a rig support vessel, while moving away from the rig, has capsized.
"A Nimrod which was flying over the Irish Sea training has been diverted to aid in the search."
Mr Mulford said Shetland coastguard scrambled its helicopter and a second North Sea helicopter was also dispatched.
An RAF Nimrod is also involved in the rescue operation.
The coastguard has appealed for help from vessels in the area.
Mr Mulford said: "This is a major incident."
"Conditions are not too bad, hopefully the weather won't be a huge factor although you never quite know. This is a fairly hostile part of the North Sea we are talking about."
Little was known about the condition of those who have been recovered.
Mark Clark, of the Maritime and Coastguard Agency, said: "At the moment we've got a mayday signal out, with an emergency signal out with all shipping in the area.
"Anybody with diving support crews, dive equipment can get on the scene for us as quickly as possible.
"We're still searching and of course the difficult situation for everybody is if they are still strapped inside the hull. And therefore we need to get to them as quickly as possible."
Neville Davis, rescue co-ordination centre manager, Shetland Coastguard, added: "So far 10 persons have been recovered. We will continue working with the assets available to us to try and locate the missing crew."
The Bourbon Dolphin had a number of roles in the North Sea, including anchor handling and towing, the installation of subsea construction blocks and operations involving remote vehicles.
A spokesman for Bourbon Offshore said: "The vessel had a problem and capsized. The crew are fully Norwegian.
"People have been rescued but there are still some people missing and there is a rescue operation under way."
A statement from rig operators Transocean said: "At 5.20pm today, the Bourbon Dolphin, an anchor handling tug supply vessel owned by Bourbon Offshore, capsized while handling anchor two of the semisubmersible drilling rig Transocean Rather.
"Two nearby vessels are providing assistance, and two helicopters have been mobilised from Sumburgh in support of rescue operations.
"We have no further details available at present but, in accordance with company policy, we have activated the Transocean Emergency Response Team who will provide support to the rig management as needed, and are maintaining contact with the appropriate emergency services.
"The first duty of the Emergency Response Team is to do everything it can to ensure the safety of all personnel on the anchor handling vessel and the rig."
Source: BBC News