NATO-led forces opened fire on a civilian vehicle in the Afghan capital Kabul on Thursday, killing a local cleric and sparking demonstrations.
Dozens of people protested in front of a military base over the killing by NATO's International Security Assistance Force (ISAF), which said it deeply regretted the death.
"In an unfortunate incident this morning, an ISAF convoy fired on what appeared to be a threatening vehicle. Regrettably, an Afghan civilian was killed during the incident," the force said in a statement.
"We later learned that the civilian was Muhammad Yonus, the Imam of Paktia Kowt Mosque in Ud-khil district. He was initially wounded in the incident but later died of his wounds," the statement added.
"I express my sincere regrets for this loss of life and convey my deepest condolences to his family," said Brig Gen. Eric Tremblay, ISAF spokesman, in the statement.
"A thorough investigation is being conducted" and the family of the cleric will be compensated, ISAF said, without giving further details.
Civilian casualties are a sensitive issue in Afghanistan, where about 113,000 international troops, mainly NATO and US, are deployed to fight an increasingly deadly insurgency.
According to a tally by the United Nations, 2,412 Afghan civilians were killed in violence including military operations and insurgent attacks in 2009, the deadliest since the 2001 ouster of the Taliban regime.