An Afghan tribal council investigating the deaths of at least 13 civilians in Kandahar province says air strikes by US-led forces were to blame.
US-led forces say their bombers were not in the area
Officials say the deaths on Tuesday night in Maroof district bring the number of civilians killed this week across the country to around 70 people.
A US spokesman denied any air strikes had taken place in Maroof.
On Thursday, Nato vowed to improve coordination with the Afghan authorities to avoid civilian deaths.
"There were no bombs drops at the place and at the time alleged," the spokesman told the BBC News website.
Earlier, provincial assembly member Janan Gulzai told the BBC Pashto service that he personally saw the dead civilians near the border with Pakistan.
Nato has been warned of 'serious consequences'
And in comments to the AFP news agency Mr Gulzai said: "We cannot accept the killing of Afghan civilians by anyone."
Mr Gulzai was a member of a provincial tribal council - or jirga - that investigated the incident.
It concluded that those killed were travelling in three cars along the same stretch of road as coalition troops near the town of Spin Boldak when the troops came under Taleban fire.
The 13 civilians were killed when coalition warplanes were summoned to bomb the area while the Taleban escaped, it said.
There have been protests across the country throughout the week staged by people angry over civilian deaths in the western province of Herat and in the east.
Many of the demonstrators called on President Hamid Karzai to resign.
Correspondents say that while the protests over civilian casualties have mostly been small, a steady stream of civilian deaths will inevitably erode support for the president and the war against the Taleban, who were driven from power in 2001.
The majority of the civilians reported killed this week - around 50 - died in heavy fighting in western Afghanistan between US-led troops and militants.
President Hamid Karzai warned afterwards that his people were losing patience over the continuing bloodshed.
The president is already facing criticism over a lack of development, widespread corruption and this year's rise in violence.