The governor of Hiiraan, Ali Jeyte Osman, said the death toll from Friday's attack could rise amid reports that up to 27 people were killed.
"Some are injured and some ran away when the shooting started. The dead bodies are still being collected, including women and children," he stated.
Mr Osman accused the militants of being at war with the civilian population in the region, accusing them of burning villages and destroying water wells.
Local clan leader Mohamed Abdirahman described the attack as horrible, adding that such an atrocity had never happed in the region before.
"These were innocent civilians who did nothing to deserve this," he was quoted by AFP news agency as saying.
Somalia President Mr. Hassan Sheikh Mohamud vowed that his government would "leave no stone unturned" in the fight against "terrorism" in the country.
In a statement al-Shabab said the lorries it attacked were carrying food supplies for a group of local fighters who have been backing the government's offensive against them.
An ongoing drought in the Horn of Africa has displaced hundreds of thousands of people in Somalia with the UN warning recently that worst was yet to come.