Prime Minister Jean Chretien pledged an investigation to answer questions about the deaths of four Canadian soldiers killed by a U.S. laser-guided bomb Thursday during a training exercise in Afghanistan.

Chretien expressed his condolences to the soldiers' families during a speech to the national Parliament in Ottawa hours after the Canadian military's first deaths in a combat zone since the Korean War. Chretien said Canada owed the troops a "debt of gratitude that is beyond mortal computation."

"We have so many questions this morning. Extensive training for combat is meant to save lives. How does this happen? In this awful case it took so many lives, and I want to assure the families and the people of Canada that these questions will be answered," Chretien said.

"President Bush has pledged full cooperation of Americans with us in the investigation that is already under way," he said. Chretien did not say what questions the investigation would pursue.

At the White House, Bush made three Thursday morning appearances without referring to the fatal accident. When asked about this, Bush spokesman Ari Fleischer said, "The president addressed it directly last night with the prime minister."

"This fight against terrorism, unfortunately, will risk the loss of life and the president very deeply regrets the loss of lives of anybody from any citizenship who is fighting with us in Afghanistan. And that extends directly to our good friends the Canadians, who lost lives last night."

The victims were identified as Sgt. Marc Leger, Cpl. Ainsworth Dyer, Pvt. Richard Green and Pvt. Nathan Smith.

Brig. Gen. Ivan Fenton did not give their hometowns, but said two victims were from Edmonton, Alberta; one from Hamilton, Ontario; and another from Halifax, Nova Scotia.

Besides the four dead, eight Canadians were wounded when a U.S. Air National Guard F-16 fighter mistakenly dropped the 500-pound bomb on the soldiers during a live-fire training exercise near the southern Afghan city of Kandahar at about 1:55 a.m. local time, Pentagon and Canadian officials said.

The jet, flying in tandem with another American F-16, had been sent out on patrol. It was unclear whether they had been given any other mission.

The pilots apparently weren't aware they were flying over an area restricted for training -- and so fire from the training exercise made them believe they were under attack, Pentagon officials said.

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

Canadian Defense Minister Art Eggleton said Thursday that one of the wounded had life-threatening wounds and the seven others were in stable condition.

Canadian forces are fighting alongside U.S. and European troops seeking to hunt down remnants of Osama bin Laden's terrorist organization and holdouts from Afghanistan's former ruling Taliban militia.

The deaths stunned the military and the public in a country that rarely sees its soldiers killed. The last soldiers to die during a Canadian military combat operation were in the 1950-1953 Korean War — though several dozen Canadian citizens serving the U.S. military were killed in the Vietnam War. There have also been 106 deaths in Canadian peacekeeping operations that date to 1948.

In Toronto, some Canadians questioned their military's presence in Afghanistan.

"Canadians are never attacked by terrorists so Canadians shouldn't be there," said Richard Sella, manager of Bistro 97. "Why are we sending our own people over there to be killed? There is no reason to be in Afghanistan."

Vyphi Vyphilin, owner of Nick's bar, said, "Four Canadians died for no reason."

But Chretien told Parliament, "The campaign against terrorism is the great global struggle for justice in the 21st century. As in all conflicts of the past, Canada has been on the front lines.

"The Canadian armed forces have set themselves apart with their valor, daring and skill," he said. "Words cannot fully express the pride that all Canadians have felt at the exemplary way they have carried out their duty."