Insurgent use of roadside bombs in Afghanistan has surged 80 percent this year as they remain the No. l killer of foreign troops, a NATO official said Thursday.

The increase since the corresponding period last year includes bombs that detonated or were found by troops before they could explode, said Canadian Brig. Gen. Richard Blanchette, a spokesman for NATO's International Security Assistance Force.

"This is very serious business for us," Blanchette told AP Broadcast in an interview from Kabul, Afghanistan.

Roadside bombs have been the primary killer in both the Afghanistan and Iraq wars. Last year, improvised devices and other roadside explosives killed 172 U.S. and coalition troops in Afghanistan. At least 31 American soldiers have been killed by roadside bombs this year, according to the Defense Department.

An American soldier was killed Thursday when a roadside bomb hit a U.S. military vehicle in eastern Afghanistan.

NATO reports that roadside bombs have caused 60 percent of the deaths in Afghanistan and severely wounded thousands of troops. The military does not release specific incident numbers, saying the data is classified.

Part of the reason for the increase, Blanchette said, is the militants' understanding that they cannot confront the military in direct action. "They are using this as a last measure," he said.

Blanchette said as the methods and technologies change, the military needs to focus on cutting off the supply system that allows bomb components to flow into Afghanistan, in many cases from neighboring Pakistan.

He said stopping the bombings is a challenge because of the variety of places militants can use them: buried along a roadside, under a road, or hidden in a wall along routes military convoys commonly use.

"Anyplace where there is a bit of a funnel," Blanchette said, "you have that risk."

U.S. and international forces train troops on how to deal with the devices, both before and after they arrive in Afghanistan.

But Blanchette said militants are constantly adjusting and finding new ways to set up and camouflage the devices.

"There's always a risk that troops might be surprised," he said.