A group of unknown gunmen opened fire at passengers of a bus travelling on the Kabul-Ghazni highway killing at least 13 civilians.

At least one of the victims is said to be a woman passenger. Two others have been critically injured in the incident which took place on the road connecting Kabul and Kandahar via restive provinces.

The motive of the midnight attack is not known as yet. Local reports suggest that the assailants stopped the vehicle on the highway and randomly sprayed bullets on the passengers after ordering them to step out of the bus.

"After the shooting, the security force arrived in the area and began clashing with the insurgents. One of the insurgents was killed while the rest fled the area," acting governor's spokesperson Abdul Majid Khogyani told Afghanistan's Tolo news.

Police have started investigations and no group has claimed responsibility for the attack so far.

Afghanistan has been battling serious Islamist insurgency led by Taliban militants and the attacks have been increasing in recent months.