The explosion was heard around Bulo Burte town, close to the main town of Beledewyne controlled by government forces and African Union (AU) troops.
"The explosion was very huge and I saw people fleeing from the area. Eight dead men were taken out of the house where the vehicle was constructed. Five more severely wounded fighters were also taken away," Xinhua quoted Ali Muse, a resident, as saying.
The military said the vehicle was intended to target the government forces and AU peacekeeping troops based in Beledweyne, 300 km from Mogadishu.