Several Canadian military aircraft and coastguard vessels combed 4,100sq km of ocean surface looking for survivors who may have drifted away from the crash site.

'Technical problem'

"Unfortunately there were no sightings whatsoever of any persons in the water," McGuire said.

The search would continue throughout Friday "until there is no chance of locating any survivors," he said, adding that search teams would cover the area "at least another time and possible three".

Reports said the charter helicopter started to turn around because of "technical problems". The gear box reportedly experienced a sudden drop in oil pressure.

The pilot radioed for help, but moments later the helicopter hit the water at 9:48am local time (12:18 GMT), McGuire said, correcting an earlier timeline of events.

The cause of the crash is still being investigated.

Two life rafts were found at the scene, but both were empty.

They may have been deployed automatically upon impact, or manually released, officials said.