A roadside bomb has ripped through an Afghan army vehicle in northeastern Afghanistan, killing six soldiers, the defence ministry said Tuesday.

The soldiers were returning to their post after a firefight with a group of militants in Alasay district of Kapisa province late on Monday.

"Unfortunately, their vehicle hit a roadside bomb which killed six soldiers," deputy defence ministry spokesman Gen. Dawlat Waziri told AFP.

"Five militants including their group leader were also killed in the firefight," he said.

No one has claimed responsibility for the attack, but roadside bombs have been the weapon of choice for Taliban insurgents in their 12-year battle to topple the US-backed central government.

Afghan forces have been playing an increasing role in the fight against the Taliban as US-led NATO soldiers progressively pull out.

Kapisa, an unstable province where French troops were stationed, lies to the northeast of Kabul close to the border with Pakistan's lawless and insurgent-infested tribal areas.

The French forces withdrew from Kapisa in 2012 and handed over security responsibility to Afghan forces.

More than 50,000 NATO combat troops still in Afghanistan are due to leave by the end of the year, and will leave the huge task of providing security to local forces.