A top Nato commander has apologised for the deaths of Afghan civilians in an air raid on Tuesday.
Gen Jones says he has expressed his concern to Mr Karzai
General James Jones said Taleban insurgents were to blame for using the villagers as cover.
Nato has confirmed that at least 12 civilians were killed in an air strike in Kandahar province, but Afghan officials say the number was about 25.
Afghan President Hamid Karzai said on Friday that he was "hurt and saddened" by the incident.
Gen Jones, Nato's supreme commander for Europe, who is in Afghanistan for a three-day visit, made the comments at a news conference at Bagram airbase in Afghanistan.
'Regretted and investigated'
He said that Taleban militants were using civilians as human shields and that in the heat of battle, when it was hard to differentiate between the two, decisions had to be made on the spot.
Eyewitnesses said the number of dead civilians was much higher
"That innocent people were wounded or killed is to be regretted and investigated," he said.
"I personally apologise for the incident, for any loss of life."
Gen Jones said he had already expressed his concern to Mr Karzai.
The deaths occurred on a day of clashes in southern Kandahar. Nato says 70 people were killed in three raids that day, the majority of them Taleban militants.
The civilians were killed during one of the raids. Eyewitnesses have suggested that the number of civilian victims was higher than 25.
Mr Karzai, who has been under mounting pressure over civilian casualties, has set up an inquiry to investigate the deaths.
In New York, a human rights group said Nato troops needed to do more to protect Afghan civilians.
"Nato's tactics are increasingly endangering the civilians they are supposed to be protecting and turning the local population against them," Sam Zarifi of Human Rights Watch said.