Helicopter crashed in Atlantic waters off Canada, A local rescue co-ordinator told news agencies that 18 people were on board at the time of the incident.
Jeri Grychowski said one survivor had been lifted from the freezing water, but the fate of the other 17 remains unknown at present.
A search-and-rescue operation is under way, involving military and civilian aircraft and ships.
The helicopter was said to have been ferrying workers to the Hibernia offshore oil platform.
It reportedly ditched 47 nautical miles (54 miles; 87km) south-east of St John's, capital of Newfoundland.
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