A helicopter carrying 20 people from an oil platform crashed on Wednesday in the North Sea, Britain's defence ministry said.
The Bond Super Puma helicopter went down off the northeast coast of Scotland, a ministry spokesman told AFP, without mentioning fatalities.
"At just before 2:00 pm (1300 GMT) this afternoon, Aberdeen Coastguard were informed of a ditched Bond helicopter about 35 miles off the coast of Crimond," the Maritime and Coastguard Agency (MCA) said in a statement.
The MCA said two Royal Air Force helicopters and a Nimrod marine patrol aircraft had been scrambled to the scene, while Aberdeen Coastguard was broadcasting a mayday signal into the area.
"A variety of vessels have immediately responded (to the mayday signal) and the nearest vessel the 'Normand Aurora' have launched their fast rescue craft to the scene," it said.
Oil giant BP said last month it had temporarily stopped using the Super Puma helicopter after one ditched in the North Sea while carrying 18 workers to an oil rig.
The 16 passengers and two crew on board escaped on rubber life rafts and were later rescued safely.