Dozens of workers were evacuated from an oil rig in Britain's North Sea after a vessel carrying radioactive waste caught fire and started drifting towards it, officials said Wednesday.

Some 52 workers were taken off the platform by helicopter late Tuesday night after a fire on the Danish-registered Parida, which was transporting material from Dounreay -- a nuclear site in northern Scotland which is being decommissioned -- to Antwerp in Belgium.

The ship's cargo was low-level radioactive waste encased in concrete, the Maritime and Coastguard Agency said.

Having lost engine power, the vessel drifted off the northeastern Scottish coast about 15 miles (25 kilometres) from the platform. But it is now at anchor and is set to be towed to shore.

"The cargo vessel Parida which was drifting unpowered on Tuesday evening is now anchored in the Moray Firth," the Maritime and Coastguard Agency said in a statement.

"A tug is on scene to assist or escort the vessel for inspection or repairs on Wednesday."

Weather conditions were "pretty rough" when the fire broke out, with eight metre waves and winds of 35 knots, Shetland Coastguard said.