It said the International Security Assistance Force (ISAF) called an airstrike on two vehicles believed to be carrying a Taliban leader and his associates, but later discovered they were transporting civilians.

"Afghan civilians were accidentally killed and wounded in Naw Zad district, Helmand province yesterday (Friday)," ISAF said in a statement, adding that it had launched an investigation into the incident.

Civilian casualties in military operations are highly sensitive in Afghanistan as coalition troops battle to curb a Taliban-led insurgency ahead of a planned handover of security to Afghan forces by the end of 2014.

The latest incident came after nine people who Afghan officials said were children collecting firewood were killed in a NATO airstrike near the border with Pakistan earlier this month.

The United Nations says Afghan civilian deaths in the war increased 15 percent to a record high last year, with insurgents responsible for three-quarters of the killings.

Helmand, the heartland of the global opium trade, is one of the biggest flashpoints in the 10 year Taliban insurgency that erupted after a US-led invasion brought down their regime in 2001.

Officials in Helmand, where mobile phone networks have been cut off for several days on Taliban orders, could not immediately be reached for comment.