The bomb, blamed on Taliban insurgents, hit a coach in Nawzad district, said a spokesman for the provincial governor.

The blast was well to the north of where Nato and Afghan troops are waging a major offensive against the Taliban.

Taliban insurgents have increasingly resorted to using roadside bombs as Nato countries have increased their troop numbers in Afghanistan recently.

The Taliban has come under pressure from the increased foreign forces acting with Afghan troops but have struck back with bombings and suicide attacks.