KABUL — A truck filled with explosives that police believe may have been destined for Kabul blew up on a highway Thursday, killing 25 people — more than half of them children walking to school. Two American soldiers died in combat as the U.S. military reported the number of roadside bombs in Afghanistan last month was nearly three times the figure for Iraq.

The attacks served as a grim reminder that the bloody conflict is widening, even as thousands of U.S. troops are being sent to Afghanistan to try to turn the tide against the Taliban-led insurgency, which has made a comeback after the Islamic extremist movement was ousted from power in 2001.

The blast occurred about 7 a.m. as police were trying to clear a traffic jam on a road in Logar province after the truck, which was loaded with timber, had overturned the night before. Suddenly, explosives hidden beneath the timber detonated, killing 21 civilians and four policemen, Interior Ministry spokesman Zemerai Bashary said.

At least 13 of the dead were children on their way to school, provincial official Kamaluddin Zadran said. Three children were missing, he added.

It was unclear why the explosives detonated. Provincial police chief Mustafa Khan said the truck overturned late Wednesday as it traveled on the main road from Logar to Kabul and militants detonated it remotely when police tried to clear the way.

However, another police official said investigators were looking into the possibility that militants were trying to smuggle explosives into Kabul and the explosives detonated accidentally. He spoke on condition of anonymity.

Kabul, the headquarters of the Afghan government where most international missions are based, is heavily guarded and has been largely spared from the violence that has rocked Baghdad for years. But rumors have been circulating that the Taliban were planning attacks in the capital ahead of the Aug. 20 presidential election.

News footage from the scene showed a huge crater in the road. Policeman Lal Mohammad said he was about 100 yards from the blast and saw "a big fire and smoke from the main road."

He ran to the scene and saw bloodied survivors and body parts littering the scene.

"I collected five bodies myself and then picked up body parts," said Mohammad. The shock wave collapsed a wall of Mohammad's home 200 years (meters) away.

Also Thursday, the U.S. military announced that two American service members were killed in a roadside bombing the day before in southern Afghanistan. No further details were released.