Four Afghan civilians were killed as a roadside bomb struck a car in Helmand province 555 km south of Afghan capital Kabul on Wednesday, Interior ministry said in a statement released here. \"The tragic incident happened in Alizai area of Nawzad district at 08:30 a.m. local time leaving all the four commuters, all civilians dead on the spot,\" the statement added. It also blamed Taliban militants for planting the mine. However, the outfit has yet to make comment.