10 of the crew were reportedly rescued from the site but the remaining five are feared to have been trapped inside the overturned hull.

Six royal Navy rescue divers were flown to the site from RAF Faslane in the UK, but search activities were called off until first light, the BBC reported.

The accident happened at about 1750 BST on the Rosebank field, about 120 kilometres north-west of the Shetland Islands.

Royal Airforce spokesman Mike Mulford told the BBC said the rescue operation 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."

Mulford said eight of the 10 rescued crew were "known to be safe", but he "did not know for sure" about the condition of the remaining two members. The rescued members of the crew were admitted to hospital in Lerwick in the Shetland Islands.

Two helicopters were scrambled to the scene immediately following the accident from the coast guard base on the Shetlands and from northern Scotland. An RAF Nimrod aircraft had also joined the search, which is being co-ordinated from the Shetlands, Mulford said.

He described the accident as "a major incident", the broadcaster reported.

The Bourbon Dolphin is owned by French marine services group Bourbon Offshore and is operated from the Norwegian port of Fosnavaag by its Norway unit. The vessel was launched in October.

Bourbon confirmed last night that the vessel had capsized and that some crew remained missing. It said the crew was entirely Norwegian. It said its Norwegian emergency team was responding to the incident.

US-based rig operator Transocean said the vessel had overturned while handling an anchor. It said two helicopters had been mobilised and two nearby vessels were assisting with the search.

It said its emergency response team was supporting the rig's crew.

Published: 12 April 2007 23:20 GMT  | Last updated: 13 April 2007 20:38 GMT