Four Afghans, including a child and two women, were killed on Saturday when U.S. forces opened fire on a car in southern Kandahar city, police said.
A man in the car also was killed when a US military convoy opened fire on the civilian vehicle, Kandahar police official Shah Agha told Reuters news agency. He said a US Special Forces convoy appeared to be involved.
A spokeswoman for Nato in Afghanistan was not able to confirm the nationality of troops involved. A statement from the provincial governor of Kandahar confirmed the casualties.
A Nato spokesman in Afghanistan said three civilians were killed and two were wounded when Nato forces fired on the car because it failed to stop when repeatedly signalled to do so.
The spokesman was unable to confirm if children were among the casualties.
The incident comes while preparations are under way for Afghanistan's presidential election run-off, which the Taliban vowed earlier to disrupt and urged Afghans to boycott.
US Army General Stanley McChrystal, the new US and Nato commander in Afghanistan, issued a directive in July stressing the importance of avoiding civilian casualties, which have undermined support for the war against the Taliban.
The Taliban had today called on Afghans to boycott next month's presidential election run-off and vowed to disrupt voting in a repeat of their threat to derail the disputed first round.
"The Islamic Emirate of Afghanistan once again urges their respected countrymen not to participate," the Taliban said in a statement, emailed to Reuters, saying the election process was being orchestrated by Washington.
"In order to make this process fail all the mujahideens will carry out operations on the enemy's centres," it said of the thousands of polling stations to be set up for the November 7th vote between president Hamid Karzai and challenger Abdullah Abdullah.
This week's announcement of the run-off removed one stumbling block for US president Barack Obama as he weighs whether to send more troops to Afghanistan to fight a resurgent Taliban.
Election officials are busily trying to prepare for the new vote, called after heavy international pressure, as Afghanistan's harsh winter approaches.
A UN-backed fraud investigation invalidated thousands of Mr Karzai's votes from the August 20th first round, pushing him below the 50 per cent mark needed to avoid a run-off against Mr Abdullah, his former foreign minister.
The Taliban, who were overthrown by US and Afghan-led forces after the September 11th, 2001, attacks on the United States, also threatened to disrupt the first round.
Sporadic attacks against candidates, election officials and polling stations failed to disrupt the process entirely but security fears contributed to a low voter turnout, especially in Taliban strongholds in the south and east.
The Taliban, which ruled Afghanistan from 1996 until they were ousted by US-backed Afghan forces in 2001, have strengthened their insurgency, with 2009 the deadliest year of the eight-year war.
The austere Islamist movement wants all foreign troops out of Afghanistan.
Security is again a major concern and election officials have warned that Nato and domestic security forces are not giving themselves enough time to secure polling centres for the run-off.
The Taliban statement warned Afghans not to leave their homes on polling day.
"The people must not take part in the elections ... all the main roads will be blocked or closed to the government and private vehicles on the day before (the poll)," the statement said in a reiteration of its first round threat.
Reuters