AFP/Kabul

Unknown gunmen Saturday killed 13 passengers travelling in two vehicles in the usually tranquil northern Afghan province of Balkh, as President Ashraf Ghani convened an international donor conference in Kabul.

No group immediately claimed responsibility for the killings of ethnic Hazara minorities, which come as Taliban insurgents ramp up attacks amid a bitter leadership transition.

"The gunmen stopped two vehicles, lined up all the male passengers and shot them dead," said Jafar Haidari, the governor of Zari district, where the incident occurred.

"They spared the life of one women who was in one of the vehicles. All the victims were ethnic Hazara minorities."

Abdul Razaq Qaderi, the deputy police chief of Balkh, confirmed the fatalities, adding that officials were investigating the incident.

The killings came as Afghan President Ashraf Ghani implored international donors for their continued support, saying the "wounded country" faced a host of security and economic challenges.

"Rebuilding Afghanistan is going to be a long-term endeavour," Ghani said at a conference of donors in Kabul attended by Western delegates and non-governmental organisations.

"Afghanistan is a wounded country. Widespread unemployment, a violent insurgency, and the advance of extremism across the region are increasing the likelihood that (our) economic reform agenda will be undone by political unrest."

Taliban insurgents are stepping up their summer offensive launched in late April amid a simmering leadership succession dispute after the confirmation of longtime chief Mullah Omar's death.

Mullah Akhtar Mansour, a trusted deputy of Omar, was named as the insurgents' new chief in late July, but the power transition has been acrimonious.

Afghan security forces, stretched on multiple fronts, are facing their first fighting season without the full support of US-led NATO forces.

NATO ended its combat mission in Afghanistan last December and pulled out the bulk of its troops although a 13,000-strong residual force remains for training and counter-terrorism operations.