The US F-16 pilot said he mistook the live fire for an attack - even though it came from a known training area.

We have so many questions this morning - I want to assure the families and people of Canada that these questions will be answered

Canadian Prime Minister Jean Chretien

Four Canadian soldiers died and another eight were injured when the 500 pound bomb struck during a live-fire training exercise near the southern Afghan city of Kandahar.

Canadian flags across the country are at half mast.

The soldiers were not only the first Canadians to be killed in the US-led campaign in Afghanistan, but the first Canadian deaths in a military campaign since the Korean War in the early 1950s.

Questions

Canada's defence chief Lieutenant General Ray Henault said the area was recognised as a training area and the aircraft were using strictly-controlled routes.

The attack came after two US F-16s reported being fired on and were given clearance by their air controller to return fire, US officials said.

No one has yet explained why neither the pilot nor the air controller were aware of what was happening on the ground.

"We have so many questions this morning," Mr Chretien told parliament in Ottawa just hours after the deaths were reported.

"I want to assure the families and people of Canada that these questions will be answered.

"President (George W) Bush has pledged full cooperation of Americans with us in the investigation that is already under way."

Mr Chretien expressed his condolences to the victims' families and said the nation owed the soldiers a "debt of gratitude that is beyond mortal computation".

Flags are flying at half mast

The Canadian prime minister questioned how troops could be attacked during a training exercise.

They were members of the 3rd Battalion of the Princess Patricia's Canadian Light Infantry, which is based near Edmonton, Alberta.

More than 700 Canadian soldiers are deployed in Afghanistan and are operating out of Kandahar.

On Monday, four American soldiers were killed while blowing up unexploded rockets, also near Kandahar.