KABUL — An explosion caused minor damage but no injuries Wednesday at the Commerce Ministry in the Afghan capital, a spokesman for the international peacekeeping force said.

The source of the explosion was being investigated, said Maj. Kevin Arata, of the International Security and Assistance Force. Canadian troops were on patrol nearby around noon when a small device exploded in an alleyway. A Para Company patrol from the 3rd Battalion, Royal Canadian Regiment said the blast was minor. The explosion slightly damaged a wall and blew out windows at the nearby office of the international charity, Save The Children.

"We still don't know if it was an accidental explosion or if it was something that was done on purpose," said Arata.

The sound of the blast was loud enough to shake the building, said Muhammad Farid, a guard at Save The Children. He said the explosion had apparently come from rocks piled in front of the building for construction.

No further details were immediately available.

While rocket and mortar attacks are common against the Afghan government and international troops outside Kabul, they are relatively rare in the capital itself, which is patrolled by the 5,000-strong NATO-led peace force.