Sarhadi Zwak, the provincial governor's spokesman, says the attack took place this morning in Badpakh district. He says four policemen were also wounded in the explosion.
No one has claimed responsibility for the bombing but roadside bombs targeting Afghan troops and security forces are a common insurgent tactic of the Taliban.
Also, Afghan civilians increasingly find themselves under fire as the US-led military coalition prepares to pull out of the country by the end of the year and Afghan forces take the lead in operations against the Taliban.