Nato has apologised for killing nine civilians in an air strike in Afghanistan's north-eastern region.
Top Nato commander Gen David Petraeus said he was "deeply sorry" for the incident, which took place in the Pech district of Kunar province on Tuesday.
Local officials say nine boys, aged 12 and under, were killed while gathering firewood, according to AP news agency.
Earlier on Wednesday, Afghanistan President Hamid Karzai condemned the killings as "merciless".
Mr Karzai warned that foreign forces would encounter "huge problems" if the "daily killing of innocent civilians" did not stop.
Gen Petraeus said Tuesday's fatalities had occurred due to a miscommunication error about the location of militants.
"We are deeply sorry for this tragedy and apologise to the members of the Afghan government, the people of Afghanistan and most importantly, the surviving family members of those killed by our actions," he said in a statement.
"These deaths should have never happened and I will personally apologise to President Karzai when he returns from his trip to London this week," he added.
Gen Petraeus said he had ordered all helicopter crews to be re-briefed on the need to keep civilian casualties "to the absolute minimum" and said that troops could face disciplinary action.
Over the weekend, an Afghan presidential team accused Nato forces of killing 65 civilians, including 40 children, in a separate incident in Kunar province, but Nato has disputed the claim.
The Nato-led International Security Assistant Force (Isaf) says most of those killed were insurgents and there were only a few civilian casualties.
Earlier this year, a human rights watchdog released a report saying 2010 was the deadliest year for Afghans since the war began in 2001.
Afghanistan Rights Monitor said the Taliban were responsible for about 60% of the 2,400 civilians killed, while US-led forces were accountable for 21%.