Five crewmen from an oil rig support boat are missing after the vessel capsized in the North Sea.
Ten people have been rescued after the Norwegian anchor-handling vessel Bourbon Dolphin capsized just 1,800m from an oil rig.
The incident happened 75 miles north west of Shetland, as the vessel was handling an anchor from the drilling rig Transocean Rather, operators Transocean said.
Eight of the 15 people on board were initially recovered and another two were winched by helicopter from the water at around 7pm. The five missing crew, all Norwegian, are thought to be be trapped in the hull of the upturned boat, which is less than a year old.
Mark Clark of the Maritime and Coastguard Agency said that as darkness fell it was imperative divers get to the scene to help with the rescue operation.
He said: "We are looking for five people and we have got the Subsea Viking, a support vessel designed for diving operations, en route to the scene and we are trying to muster divers. We may have to get them from the mainland and fly them out or there may be some on rigs.
"We cannot see anyone therefore we can only assume they are under the hull. If people are under the hull then we don`t know how much air they have or whether they have managed to struggle into their survival suits - and the water is around five degrees at the moment."
It is not known whether the missing crew members had been wearing survival suits or life jackets at the time of the incident.
A coastguard helicopter and a North Sea Rescue helicopter were scrambled to the scene and an RAF Nimrod is providing overhead cover. Three vessels, Olympic Hercules, Viking Victory and Highland Valour are also assisting with the search.
A spokesman for Bourbon Offshore said: "The vessel had a problem and capsized. The crew are fully Norwegian. There were 15 people on board. People have been rescued but there are still some people missing and there is a rescue operation under way."
