There was little that could have been done to prevent the insurgent bombing that killed four Canadian soldiers in Afghanistan, Canada's top soldier says.

Chief of Defence Staff Rick Hillier said Saturday in Ottawa that the device that killed the soldiers had "considerable explosive power."

"What exactly caused that explosion, we're in the process of determining, and we will determine it, but it was huge," he said at a news conference. "I've seen a picture of the crater, it was deep, it was significant."

Hillier said there is little that can be done to stop determined insurgents from attacking Canadian troops. "You cannot reduce the risk to zero," he said.

Hillier also defended the Mercedes-built G-wagon the soldiers were traveling in north of Kandahar when they were attacked, saying it is a "very good" vehicle that has saved the lives of Canadian soldiers.

"Any armoured vehicle can be defeated if you pile enough explosives into one location and manage to detonate it at the appropriate and specific point in time," he said.

The soldiers were killed in the Gumbad region of Afghanistan, about 75 kilometres north of Kandahar, when their vehicle was struck by an improvised explosive device at about 7:30 a.m. local time, said CTV's Sarah Galashan, reporting from Kandahar.

Three of the men died instantly. Medics had the fourth airlifted to hospital but he died before he arrived.

The military has identified the men: Cpl. Matthew Dinning, Bombardier Myles Mansell, Lieut. William Turner and Cpl. Randy Payne.

Dinning was born in Richmond Hill, Ont., and was stationed in Petawawa; Mansell was born in Victoria B.C. where he was also stationed; and Turner was from Toronto but was stationed in Edmonton. Payne was born in Lahr, West Germany and was stationed at CFB Wainright.

The men were part of a mixed convoy of light armoured vehicles and G-wagons returning to Kandahar from Gumbad. They had been stationed at a small forward operating base established in February and nicknamed the Red Devil Inn, Galashan said.

Immediately after the attack, the commanding officer of Alpha Company, 1st Battalion of the Princess Patricia's Canadian Light Infantry, led a heavily armed patrol through the area in a show of force.

Maj. Kirk Gallinger also met with village elders to gather any information he could about those responsible for the attack.

Alpha Company -- known as the Red Devils -- had soldiers stationed near the blast site at a platoon house, which has been occupied by Canadians since early February.

During his news conference at the main Kandahar base on Saturday morning, Brig.-Gen. David Fraser said he knew two of the soldiers personally (they were part of his personal protection team), and praised their efforts in the region.

"Those soldiers were conducting operations to enhance the security of the area," Fraser said. "I had actually been in the area yesterday myself, talking with the local leaders. (They) had indicated that security had improved since we arrived in the area."

Fraser, who had been to the Gumbad area just the day before, said an investigation into the attack is underway. He pledged troops will continue striving to establish peace in the region.

"Everybody here today is sadder, but we've redoubled our efforts to make sure we provide the hope and security and stability that Afghans expect from the international community," Fraser said.

Hillier said two of the soldiers were reservists, but added the military ensures all soldiers are fully trained and prepared for this mission.

The bodies of the fallen troops will return to Canada on Tuesday, he said.

Minimizing risk

U.S. Army Lt.-Col. Tom Christianson (ret.) told CTV News Canadian soldiers would have swept the area before using the road.

"What we don't know is the interval and how long it was between the time that the road was swept and in fact the Canadians moved down that road," he said.

Christianson said varying operation movement could help thwart insurgent attacks.

"The most important thing we can do, including vigilance and de-mining operations ... which were already conducted, is vary the interval between movement down that road," he said.

"In other words, you don't always move down at 8:30 a.m. or 10:30 a.m. or 12:30 p.m. You might move at night, you move at various times in order not to allow the enemy to even place the mines."

Rocket attack

Early Sunday morning in Afghanistan, the airbase near Kandahar came under attack, believed to come from two 107-mm rockets. No injuries or significant damage was reported.

"Shortly after 2 a.m., we heard two rapid explosions, we hit the ground, grabbed our flap jackets and our helmets and made our way to a shelter, where we stayed for about 45 minutes until we were given the all-clear," Galashan said.

Coalition helicopters were scrambled within minutes of the explosions and the base remained locked down for the better part of the night.

Prime Minister Stephen Harper's statement

Prime Minister Stephen Harper issued a statement praising the valour of the soldiers and the work the troops are doing in Afghanistan.

"These men were working to bring security, democracy, self-sufficiency and prosperity to the Afghan people and to protect Canadians' national and collective security. We will not forget their selfless contribution to Canada," Harper said in the statement.

"On behalf of all Canadians, I extend our deepest condolences to the families, friends, and co-workers of these four brave men. Our thoughts and prayers are with them."

Eyewitness account

Richard Fitoussi, a freelance journalist who was traveling with the convoy, described the attack to CTV Newsnet.

"There was a very loud concussion-like explosion that we felt inside the vehicle that we were in," Fitoussi said. "The convoy stopped and there was some panicky sort of suppressing gun fire. I thought for a minute that we were under small arms fire, but it was the convoy that had laid down suppressing fire as a warning to anyone that was in the surrounding hills."

A "strange calmness" took hold immediately after the explosion, but that it quickly turned into action and a tense sense of panic as troops assessed the damage and attempted to help the surviving soldier, he said.

Fitoussi said he was proud to say he had become friends with one of the fallen soldiers, William Turner, during his time embedded with the troops in Gumbad.

"He was the one fellow who sort of took me under his wing as soon as I got here," Fitoussi said. "The troops are a little skeptical of journalists... but Bill Turner was a really helpful guy and was well-loved by this unit."

Death toll climbs

Galashan said the most recent tragedy represents the largest loss of Canadian soldiers involved in combat operations since Canada fought in the Korean War.

The loss of the four soldiers Saturday was the single biggest loss in Afghanistan since four years ago when four soldiers were killed and eight others wounded in a "friendly fire" incident involving a U.S. fighter jet near Kandahar on April 18, 2002. The Canadians were on a night live-fire exercise at the time.

Sixteen Canadians, including diplomat Glyn Berry, have now lost their lives in Afghanistan since 2002. Half the deaths come from attacks this year. There are 2,200 Canadian troops currently serving in Afghanistan.

Saturday's attack came a day after Pte. Robert Costall was honoured in a memorial service in Edmonton.

Costall, 22, of the First Battalion of the Princess Patricia's Canadian Light Infantry, died last month during a firefight with Taliban insurgents at Forward Operating Base Robinson, located about 110 km northwest of Kandahar in the Sangin district of Helmand province.

With a report from CTV's Sarah Galashan