Gunmen killed 13 passengers travelling on a bus towards restive southern Afghanistan on Tuesday.

Gunmen killed 13 passengers travelling on a bus towards restive southern Afghanistan on Tuesday, officials said, the latest attack hitting civilians in the country's still-bloody conflict.

The killings in Wardak province, which lies close to the capital Kabul, underline Afghanistan's fragile security situation as President Ashraf Ghani holds talks in Washington.

Ataullah Khogyani, the spokesman for the provincial governor, told AFP a group of gunmen opened fire on a bus in Wardak around 1:00 am local time (20:30 GMT Monday), killing 13 people including one woman.

The bus was heading for the restive southern city of Kandahar, once a bastion of the Taliban.

Mohammad Ali, the deputy governor of neighboring Ghazni province, confirmed the incident, saying the gunmen picked their victims and shot them one by one.

Both officials said the motive for the attack was still under investigation.

Last month, masked gunmen abducted 30 people from a bus in Zabul province. They have still not been recovered.