"They were travelling in Bolan, an area on the outskirts of Lashkar Gah, at around mid-day. Their vehicle touched off an improvised explosive device (IED)," Xinhua news agency quoted an official as saying.
The official blamed Taliban militants for planting the IED.
The Taliban group has been using IEDs to target security forces but the lethal weapons also inflict casualties on civilians, according to military officials.
More than 3,540 civilians were killed and over 7,450 injured last year as violence spread in different parts of the country, according to figures released by the UN mission in Afghanistan.
Of the casualties, 713 civilians were killed and more than 1,650 injured by IEDs last year.
