KANDAHAR, Afghanistan — Canadian soldiers shot and killed an Afghan civilian and wounded three others Friday after the car they were in failed to heed orders to stop as it sped towards a team of troops who were defusing a roadside bomb.

Canadian Forces spokesman Mario Couture said soldiers were called in by Afghan National Police to defuse a bomb in a residential area.

They'd cordoned off a secure area when a vehicle breached the perimeter at high speed.

The driver of the vehicle failed to heed shouts, hand signals and warning shots, Couture said. Fearing a suicide bomber, he said two soldiers opened fire.

"It was obvious there was a cordon," Couture said. "There were soldiers waving. They had many chances to stop the vehicle because there were many signals used."

He said the vehicle was driving at high speed and showed no intention of slowing down -- telltale signs of past suicide attacks.

The Canadian soldiers had parked their armoured vehicles across the roads, and soldiers were on patrolling on foot.

"They followed all the proper procedures," said Couture, who described the split-second decision to open fire on a potentially threatening vehicle as one that soldiers have to make under intense pressure.

"It's a decision you have to make very quickly. It doesn't leave you much choice but to open fire, to at least immobilize the vehicle and that's what they tried to do.... But until that vehicle is stopped, it's still a threat."

No explosives were found in the vehicle, but Couture said the military is not convinced the men involved are not insurgents.

"Although we didn't find any explosives in the vehicle, we're still not convinced these guys were not testing our procedures on how to breach our perimeter."

The injured men were taken to Mirwais Hospital in Kandahar.

Troops remain on high alert in southern Afghanistan, where both NATO troops and insurgents have been very active so far this summer. The U.S. and Britain have both undertaken major offensives in neighbouring Helmand province to rout out the Taliban.

Afghan National Army corps commander Shair Mohammad Zazai said Saturday that insurgents, too, are out in force ahead of next month's presidential and provincial council elections.

"They just want to prove that they are present," he said.

A massive influx of U.S. troops to Afghanistan -- in particular to the hot spots of Helmand and Kandahar provinces -- has led to a surge in counter-insurgency operations.

The Afghan National Police and the National Investigation Service are looking into the incident.

Civilian deaths have severely hampered NATO forces in their efforts to win over Afghans in the nearly eight-year war.

Earlier this month, the new commander of U.S. and NATO forces in the country, Gen. Stanley McChrystal, issued a directive to "mitigate that risk wherever possible."

The directive asks troops to limit their use of force -- particularly air strikes -- in locations likely to produce civilian casualties.

Civilian casualties, it said, "in the long run make mission success more difficult and turn the Afghan people against us."

But the directive does not prevent troops from protecting their own lives.

Couture said the Canadian soldiers involved in the incident followed procedure and were ultimately "forced to open fire."

"It's one of those things where if you don't shoot, it could be you. It's a few-seconds decision of what to do."