"The blast hit a pick-up truck in the Arghistan district of Kandahar province, killing four women, four men and five children," Abdul Raziq, Kandahar provincial police chief said.
Raziq said the victims were travelling to the funeral of two people who were killed by a similar blast a few days ago.
"A motorbike earlier hit a roadside bomb that killed two people, and today's victims were travelling to attend that funeral when unfortunately their vehicle struck another bomb," he said.
Nine people were injured in the blast, some of them suffering life-threatening wounds, officials said.
It was the latest violence since the Taliban launched their annual "spring offensive" on April 27, opening a crucial period for the country in its offensives against the insurgents.