Also yesterday, 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.

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.

Yesterday's blast occurred about 7 a.m. as police were trying to clear a traffic jam on a highway 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. *