KABUL : A massive bomb blast in central Afghanistan killed 25 people including 13 primary school students Thursday, destroying shops and scattering pieces of the vehicle that carried the explosives over a huge area. Another bombing in the south killed two American troops.

The bomb in central Afghanistan was detonated after a truck carrying timber overturned, killing 21 civilians and four policemen in Logar province, south of Kabul, ministry spokesman Zemerai Bashary said. At least 13 of those killed were children from nearby schools, said Kamaluddin Zadran, a provincial official.

Another two schoolchildren were wounded and three others were missing, Zadran said.

Provincial police chief Mustafa Khan said the truck overturned late Wednesday as it traveled the main road from Logar to Kabul.

After police arrived to clear the road Thursday morning, militants apparently remotely detonated a bomb planted in the back of the truck among the timber, he said.

The power of the blast in Mohammad Agha district, close to shops that collect milk from farmers, sent truck pieces flying more than a mile (2 kilometers), said a second police official, who spoke on condition of anonymity because he was not authorized to speak to the media.

AP Television News footage from the scene showed the explosion left a huge crater in the road. People collected the remains of the dead, wrapping them in white and colored shrouds. Nearby mud houses had collapsed. Twisted and charred remains of a police vehicle caught in the blast were loaded onto a truck.

Lal Mohammad, a local police officer, was working his land about 100 yards (meters) away when the explosion happened.

"I saw a big fire and smoke from the main road," Mohammad said.

He ran toward the site of the explosion, and saw dead people and body parts strewn around.

"I collected five bodies myself and then picked up body parts," Mohammad said.

The explosion was so strong that a wall in Mohammad's house, about 200 yards (meters) away, collapsed.

Two American troops were killed in another roadside bombing incident Wednesday in southern Afghanistan, said Capt. Elizabeth Mathias, a U.S military spokeswoman, without providing any other details.

As of Wednesday, at least 647 members of the U.S. military had died in Afghanistan, Pakistan and Uzbekistan as a result of the U.S. invasion of Afghanistan in late 2001, according to the Defense Department. Of those, the military reports 480 were killed by hostile action.

Roadside and suicide bombings are the militants' weapon of choice in Afghanistan, and the number of such attacks have spiked this year, as thousands of additional American troops joined the fight.

In June, there were 736 reported incidents involving improvised explosive devices that killed 23 coalition troops and wounded 166 other foreign troops, according to the Pentagon. In May there were 465 incidents that killed 12 foreign troops. The majority of victims have been civilians.

In the southern province of Zabul, Afghan and coalition troops battled Taliban militants who attacked a government center in Suri district early Thursday. Fifteen insurgents were killed and another was detained, said provincial police chief Abdul Rehman Sarjang.

No casualties were sustained among Afghan and foreign troops, Sarjang said.

Southern Afghanistan is the center of the Taliban-led insurgency. The hard-line Islamist militia has made a violent comeback in recent years since the 2001 U.S. invasion that ousted their government.