Nato's secretary general has called for an investigation into the killings of 25 civilians in an air strike in the Afghan province of Helmand.
Afghan support of international forces is crucial, Nato says
Jaap de Hoop Scheffer said civilian deaths were "always a mistake".
The Nato-led force (Isaf) said it was checking reports that it had killed a small number of civilians as it responded to a Taleban attack.
President Hamid Karzai told the BBC this week that civilian deaths caused by foreign forces would have to stop.
If not, Mr Karzai warned that Afghans might turn against countries with a military presence in Afghanistan, but added that people were still grateful for that involvement.
Speaking in Quebec City, Canada, Mr de Hoop Scheffer said no Nato, coalition or Afghan soldier would knowingly take aim at a civilian, and accused the Taleban of using civilians as human shields.
"Each innocent civilian victim is one too many," he said. "Unfortunately it happens."
"It's important to avoid these mistakes because we must keep the support of the large majority of the Afghan population."
Speaking to the BBC's correspondent in southern Afghanistan, people from the village of De Adam Khan, near the town of Gereshk in Helmand, said heavy bombings of the area had resulted in the civilian deaths.
They said nine women and three children were among the 25 killed.
The accounts were backed by the provincial police chief, Mohammed Husain Andiwal, who said about 20 Taleban were also killed in the strike.
Mr Andiwal said Taleban fighters attacked Nato forces first but alleged that foreign forces had launched air strikes without consulting with their Afghan counterparts.
Isaf said its forces were attacked on Thursday night near Gereshk and responded with small arms fire and an air strike.
It said that up to 30 insurgents were believed to have occupied a compound and that most of them were subsequently killed.
In the neighbouring province of Uruzgan, Isaf has said that days of fighting appeared to have caused civilian deaths, some of which might have come from air strikes against Taleban insurgents.
There are two international missions in Afghanistan: Isaf, with 37,000 troops from 37 countries including the US. Its aim is to help the Afghan government bring security, development and better governance.
The US-led coalition - under the banner of Operation Enduring Freedom - is a counter-terrorism mission that involves mainly special forces.
The south of the country has this year seen the worst violence since the Taleban were ousted from power in 2001 by US-led troops.