"Yesterday, a Royal Canadian Navy helicopter on a NATO mission, carrying six members of the Canadian Armed Forces, went down with all hands in the Ionian Sea off the coast of Greece," Trudeau told a news conference.

"One casualty was recovered and five are missing," he said.

The Cyclone Sikorsky CH-148 helicopter was returning to HMCS Fredericton after a training mission when contact was lost, the chief of the defense staff, General Jon Vance, said.

Defense Minister Harjit Sajjan added the cause of the crash was "unknown," but that an automatic beacon was discovered in the waters, and the helicopter's cockpit voice and flight data recorders had been recovered.

The Canadian frigate and submarine-hunting helicopter were 100 days into a NATO mission, along with Greek and Turkish ships, aimed at deterring Russia.

More than 900 Canadian soldiers are deployed throughout Eastern Europe as part of Operation Reassurance -- Canada's largest current international military deployment.

Vance said a "very sizeable debris field" has been found and that a search continues in the 3,000-feet deep sea.

He also confirmed that the body of Sub Lieutenant Abbigail Cowbrough has been recovered from the crash site.

Her father Shane Cowbrough earlier said in a social media post that he was "broken and gutted" by the loss of his eldest daughter. "There are no words," he said.

HMCS Fredericton was scheduled to return to its home port of Halifax, Canada in July.

During the crash investigation and "to rule out that there's a fleet-wide problem," all of the Canadian military's relatively new Cyclone helicopters would be grounded temporarily, Vance said.