Armed men have killed 13 passengers travelling on a bus towards southern Afghanistan, officials say, the latest attack to target civilians in the country's conflict.

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

With the spring fighting season about to begin, Ghani has asked the US for "flexibility" as it pulls out its remaining 10,000 troops by the end of 2016.

Ataullah Khogyani, spokesperson for the provincial governor, told AFP a group of armed men opened fire on a bus in Wardak around 1am local time (2030 GMT Monday), killing 13 people including one woman.

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

Mohammad Ali, the deputy governor of neighbouring Ghazni province, confirmed the incident, and said the attackers picked their victims and shot them one by one.

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

Last month, masked attackers abducted 30 Shia members from the Hazara ethnic group from a bus in Zabul province. They have still not been recovered.

NATO ended its combat operation against the Taliban in December after 13 years of war, leaving Afghan forces to deal with the fighters themselves.

A smaller contingent of foreign troops, most of them American, is staying on for training and counter-terror operations.

But there have been fears that without the military muscle of the US-led NATO coalition behind them, Afghan forces could struggle to quell the campaign.

'Unsustainable' rate

The Pentagon insists Afghan forces are holding their own in the fight, but senior US officers have voiced concern that they are sustaining casualties at an "unsustainable" rate.

The US was due to reduce its 10,000 troops to 5,500 by December, but that number is expected to be reassessed after American commanders appealed to keep more troops on the ground.

Before leaving for Washington, Ghani warned of a "difficult" spring fighting season and said his government had been working hard to create conditions for talks with the Taliban.

But efforts are proving difficult despite Ghani's attempts to get influential regional powers like Pakistan on board, as the Taliban continue to impose tough conditions for negotiations.

There is also mounting concern about the influence of the Islamic State of Iraq and the Levant (ISIL).

The Middle East-based group has not formally confirmed it is operating out of Afghanistan, though Pakistani and Afghan commanders have pledged their allegiance in recent months.