The Canadians were killed when the vehicle they were travelling in hit an explosive device on April 8, Nato's International Security Assistance Force (Isaf) said. It is the worst single incident for the Nato-led force since 2005.

Canadian Prime Minister Stephen Harper confirmed the deaths in the town of Verlinghem, France, while on a visit to mark the 90th anniversary of the World War I Battle of Vimy Ridge.

Addressing a group of veterans, Mr Harper drew parallels with World War I and the challenges faced by troops in Afghanistan today.

Canadian and British troops make up most of the Isaf forces in the south.

Last year saw the worst fighting in Afghanistan since coalition troops ousted the Taleban in 2001 with some 4,000 people believed to have been killed - about a quarter of them civilians.

About 100 insurgents have been killed in it so far.