Matt Bingham, a Glasgow-based journalist, told Al Jazeera that as oil rigs, the potential for devastation should these threats be realised was huge.

"It's a major operation, both for the RAF [UK Royal Air Force] and civilian helicopters.

Heightened alert

"We're talking around 500 plus people ... a fair distance off coast so the safest and quickest thing for the rescue services to do is to move them onto neighbouring rigs about 25 miles away.

"In the meantime, a bomb disposal unit is on its way to the rig, to deal with this apparent threat," he said.

"If there is the slightest possibility that there might be a device being on one of these rigs then they can't take any chances at all, and that is obviously what they're doing," he said.

Fourteen air force and coast guard helicopters were reportedly being sent to the rig to collect its 539 occupants.

"We can confirm that a number of Royal Air Force helicopters are involved in a police investigation led by the ... police," a defence ministry spokesman said. 

A total of 539 people had been on the Safe Scandinavia unit, an accommodation area adjacent to the oil rig.

When they were made aware of the security alert they all gathered to the walkway which links the accommodation unit to the rig.