"The incident took place in Marjah district at around 6:00 p.m. local time when a minibus touched off an Improvised Explosive Device (IED) in a main road, leaving 14 aboard the vehicle killed and five others wounded," provincial government spokesman Omar Zwak told Xinhua.
Several women and children were among the victims, the official said, adding that an investigation was launched into the incident.
The official blamed the Taliban militant group for the attack.
The Taliban has largely been using home-made IEDs to launch suicide attacks and roadside bombings against security forces, but the lethal weapons also inflict casualties on civilians.
In the first four months of 2015, more than 1,000 civilians were killed and over 1,900 others wounded in conflicts and Taliban- led insurgency, which showed a 16 percent increase over the same period last year, according to official figures released by the UN mission in the country.