When John Beerenfenger heard that his only grandson had been killed yesterday in Afghanistan, he said a silent prayer. Then wept for hours.

His grandson wouldn't approve, he thought to himself. Even though Cpl. Robbie Beerenfenger's wife had just given birth to their fourth child, he didn't think twice when he got the call to serve in Kabul.

He believed in this mission and his sense of duty drowned out any worries he had about leaving his wife, Tina, and their children behind at CFB Petawawa.

"Everyone in the family is devastated. We're still in tears," the elder Beerenfenger said from his home in Carleton Place, near Ottawa.

That sadness echoed across the country as Canada mourned the deaths of Beerenfenger, 29, and Sgt. Robert Short, 42, killed when their jeep was blown up by a land mine or unexploded shell while on routine patrol in the foothills southwest of Kabul.

Three other Canadians Master Cpl. Jason Hamilton, Cpl. Cameron Laidlaw and Cpl. Thomas Stirling were in a jeep following closely behind the lead vehicle and are recovering from "non-life threatening" injuries in the massive explosion.

All the soldiers were members of Para Company, 3rd Battalion, of the Royal Canadian Regiment, based in Petawawa.

In spite of the tragedy, the federal government vowed the soldiers' deaths would not weaken its commitment to the international peacekeeping mission in Afghanistan or lessen the resolve of the nearly 2,000 soldiers still serving there.

Defence Minister John McCallum said the Canadians, part of the NATO-led International Security Assistance Force (ISAF) in Kabul, will not be deterred.

"Let me be clear on one thing," a sombre-looking McCallum declared at an Ottawa news conference. "The mission in Afghanistan is fundamental to Canada's security. Even though it is not immediately evident, when our soldiers patrol the streets of Kabul, they are also keeping the streets of Canada safe."

Prime Minister Jean Chrétien expressed his condolences.

"I was deeply saddened to learn that brave Canadian soldiers had been killed and wounded in the line of duty in Afghanistan," he said.

"Canada has, resolutely and without hesitation, joined all civilized nations in the international war on terrorism. The news today is a painful reminder that defending our values and doing our duty can come with a very high price."

The blast happened at 1:25 p.m. local time on a route Canadian engineers had swept for landmines on Wednesday, according to Maj.-Gen. Andrew Leslie, commander of Canadian forces in Afghanistan. The soldiers were travelling in Iltis vehicles, which provided almost no protection against the huge explosion.

Leslie said the incident was under investigation. It was not yet clear whether the explosion was caused by something buried that surfaced through erosion or if it was a deliberate attack "planted by those who wished to kill ISAF soldiers."

"The explosion caused a very big hole," Leslie said. "Every indication is that it was triggered by the vehicle either striking, driving over or being adjacent to the device."

Leslie said the troops were using the unarmoured vehicles because larger carriers cannot navigate Kabul's narrow, twisting streets.

At Camp Julien, where nearly 2,000 Canadian soldiers are stationed, the mood was sombre.

"Sometimes there is a price to pay for trying to help people. They died doing their duty," Leslie told the soldiers.

The incident marked the first casualties suffered by Canada since Ottawa sent troops to join the 5,500-strong peacekeeping force in August. But it came at a time of rising violence there.

The Taliban, ousted in late 2001 by a U.S.-led coalition for providing refuge to Al Qaeda terrorists led by Osama bin Laden, is stepping up attacks on peacekeepers and Afghan government officials.

Leslie said patrols aimed at keeping order in Kabul had resumed and must continue.

"The Taliban and Al Qaeda want us to retreat to our camps or run away," Leslie said. "And neither, of course, is an option for our soldiers."

For soldiers at Canadian Forces Base Petawawa, news of the deaths of their comrades came as a shock, the Star's Bruce Campion-Smith reports.

Pte. Andrea Smart was among the soldiers summoned to the base cafeteria to be told the news. "It was pretty upsetting," said Smart, whose fiancé Cpl. Ani Nehme is among those serving in Kabul. "Do you have any idea what its like waiting, not knowing?"