Relatives carry the body of an Afghan schoolboy who was killed by an explosion in Nangrahar province in Afghanistan on Wednesday. (Rahmat Gul/Associated Press)

An explosion tore through a group of children gathered around foreign soldiers visiting a U.S.-funded road project Wednesday, killing four kids and a policeman and wounding scores, including at least three American troops, officials said.

The Afghan Interior Ministry said in a statement that the blast in Nangrahar province in Afghanistan's east occurred when a passing police vehicle hit a mine. The ministry called it a terrorist act, implying the mine had been planted by insurgents.

Adjman Pardes, chief of the province's health department, said four children and a policeman died. He also told The Associated Press that 81 people, the vast majority of them schoolchildren, were wounded.

Ahmad Zia Abdulzai, the spokesman for the provincial governor, said earlier that the wounded included three U.S. soldiers. NATO's International Security Assistance Force said nine of its soldiers were wounded, but could not specify their nationalities.

Multiple explosions across Afghanistan

Abdulzai said the soldiers were visiting a road construction project funded by the United States.

The blast occurred at about 10 a.m. local time, as children were heading home from school; many Afghan elementary schools work on three shifts a day, with the first beginning in the early morning.

Children frequently cluster around troop contingents, excited by curiosity and the hopes of receiving small treats.

In a separate attack in the province, four Afghan policeman were killed when a remote-controlled bomb blew up their vehicle in the Khagyani district, Abdulzai said.

Also Wednesday, at least 15 people were injured in an explosion at a market in Khost province in eastern Afghanistan, said provincial acting governor Tahr Khan Sabari. The blast occurred outside a shop in Khost city selling cellular telephones and music cassettes; the Taliban oppose most forms of secular music and Sabari speculated the insurgents chose the shop as a target for that reason.

Conflict claims over 1,000 kids: rights group

The deaths of civilians, especially children, are an increasingly sensitive issue in the Afghanistan conflict. On Wednesday, the independent human rights watchdog group Afghanistan Rights Monitor said more than 1,050 children under 18 died last year in war-related incidents.

The group said about two-thirds of the young victims died at the hand of insurgents, including several murdered on suspicion of spying. But it also criticized Afghan and international forces, pointing particularly to the alleged deaths of eight children in an operation involving foreign troops last month in Kunar province.

NATO claims those killed in the operation were insurgents, but ARM said in a statement that it appeared to be a "crime against civilian people."

Aside from outright killings of children, the insurgents are endangering countless others by "widespread and systematic attacks on aid workers, humanitarian convoys and facilities [that] deprived thousands of children from lifesaving services such as food aid and immunization against deadly diseases," ARM said.