Three U.S. soldiers also were killed by roadside bombs, the American military said, two in southern Afghanistan and one in the east.

The incidents followed a pattern of escalating violence in widely scattered areas of Afghanistan.

The truck blast took place in Lowgar province. The vehicle, traveling on Afghanistan's main north-south highway, apparently ran off the road and overturned before dawn. When police and civilians approached after daybreak and tried to right the truck, it blew up, the Interior Ministry said.

Most of the dead were civilians, including at least 13 students from a nearby primary school, local officials said. The thunderous blast left a huge crater in the highway, hurled debris over a wide area, and collapsed several nearby shops and homes.

The truck was piled high with a load of timber, with the payload of explosives buried underneath, police said. Officials were trying to determine if it was triggered remotely when police and other help arrived or if it detonated when the truck was moved.

Ghulam Mustafa, the Lowgar police chief, said the truck's driver had disappeared, and officials were looking into the possibility that the Taliban had been transporting the explosives to the capital.

Kabul has been relatively calm lately. However, an explosion of this size and strength in a crowded area such as the capital likely would have caused many more deaths and injuries.

Over the last year, Lowgar gradually has become a hotbed of insurgent activity, alarming Western military officials and fostering the impression that the Taliban was tightening a noose around Kabul. Travelers on the main road south out of the capital face constant danger of ambush, abduction, or banditry when passing through the province.

In an effort to weaken extremists' grip on the area, U.S. troops from the 10th Mountain Division, based at Fort Drum, N.Y., have been deployed in Lowgar and adjoining Wardak province for about six months. In addition, a "village guards" initiative has been set up in the two provinces, which Afghan and Western authorities say eventually may be expanded countrywide.

Under the program, local residents work as a kind of auxiliary police force to try to keep the insurgents from overrunning remote villages.

The latest bloodshed coincided with a major U.S. military offensive in southern Afghanistan's Helmand province, which is a center of both the insurgency and the drug trade.

In the last week, about 4,000 U.S. Marines have secured the lower Helmand River Valley, and commanders say they plan to hold the territory with a string of new outposts and a concerted effort to reach out to mistrustful tribal elders and village leaders.

British, Canadian, and Dutch troops are also fanned out across much of the south, and their casualties have spiked in the last few weeks. An increase in troop fatalities usually occurs during the summer months, when insurgents are better able to move through mountain passes on the border with Pakistan, bringing in fighters and weapons.