The HMCS Fredericton, participating in NATO's Operation Reassurance meant to bolster security in central and eastern Europe, lost contact with a CH-148 Cyclone helicopter on Wednesday during a training exercise.

"They are all heroes," Trudeau said in a news conference.

"The cause of this accident is unknown at this time," Defence Minister Harjit Sajjan said, adding that the aircraft's data and voice recorders have been recovered.

General Jonathan Vance, Chief of the Defence Staff, said that search efforts continued and that there was a "very sizable debris field" in an area of the Ionian Sea where the aircraft crashed.

Vance said the body that was recovered was that of Abbigail Cowbrough. Tanya Cowbrough, the mother of Abbigail, commented on Facebook: "Nothing can replace her."

"I am broken and gutted," Shane Cowbrough, Abbigail's father, wrote on Facebook. "There are no words. You made me forever proud. I will love you always, and miss you in every moment. You are the bright light in my life taken far too soon."

It would be the country's single deadliest military tragedy in 13 years if search efforts do not locate any survivors. In July 2007, six soldiers were killed together by a roadside bomb in Afghanistan.

After the crash, Canada's military put its Cyclone helicopters on an "operational pause", Vance said, until a "fleet-wide" problem can be ruled out. The Cyclone is made by Lockheed Martin's Sikorsky unit.

Approximately 2100 Canadian Armed Forces members are deployed around the world, and 915 people are part of the North Atlantic Treaty Organisation's Operation Assurance.

Australian Associated Press