Nine Afghan boys were killed while collecting firewood on Tuesday in what has been described as one of the worst cases of mistaken identity by the International Security Assistance Force.

NATO issued a statement Wednesday apologizing for the killings in Darah-Ye Pech district in Afghanistan's Kunar province. The victims were aged nine to 15 and included two sets of brothers. A tenth boy was injured but survived the aerial attack by helicopter gunners.

The boys had apparently been collecting wood to heat their mountain homes, the New York Times reported.

An anti-American demonstration was held Wednesday by enraged Afghans in the village where the boys lived, the paper said.

"We were almost done collecting the wood when suddenly we saw [two] helicopters come," the boy who survived told the Times. After hovering over the boys, the helicopter gunners shot his friends one by one, he said.

"We are deeply sorry for this tragedy and apologize to the members of the Afghan government, the people of Afghanistan and most importantly, the surviving family members of those killed by our actions," Gen. David H. Petraeus, commander of ISAF, said in a statement.

"These deaths should have never happened and I will personally apologize to President [Hamid] Karzai when he returns from his trip to London this week."

Civilian casualties have long been a source of tension between the U.S.-led international force and Karzai.

The Afghan president condemned the deaths, and said in a statement, "Is this the way to fight terrorism and maintain stability in Afghanistan?"

An investigation is being carried out to determine how the boys were mistaken for insurgents and to try to prevent similar incidents in the future, Petraeus said. If warranted, he said disciplinary action could be taken.

"I have ordered all ISAF leaders and members of ISAF attack helicopter crews to be re-briefed on the tactical directive, reinforcing the need to be sure we protect the lives of innocent Afghans as we pursue a ruthless enemy," Petraeus said.

With files from The Associated Press