The British coast guard says workers have been evacuated from a North Sea oil rig after a reports of a spill.

The Maritime and Coast Guard Agency says 56 nonessential staff have been airlifted off the Heather Alpha rig, about 60 miles east of the Shetland Islands. Thirty-four people remain on board.

Coast guards say no emergency rescue craft are involved in Friday's evacuation.

They say reports suggest 11 to 22 tons of crude oil have leaked from the rig.

Operator Petrofac confirmed it was investigating the incident and said emergency procedures had been activated as a precaution.