Officials said they did not know who set off the bomb, but such attacks are common in the country's remote tribal regions, where militants from both Pakistan and neighbouring Afghanistan are active.
The vehicle was hit in the Jandol area while taking people from a border village to the town of Munda early in the morning, said Ejaz Abid, police chief in Lower Dir district where the attack took place.
He said the bomb was planted in a dirt road and apparently detonated by remote control.
The injured were rushed to a hospital, where some remained in serious condition.
Local government official Mahmood Aslam said two children were among the dead. He said the vehicle was not carrying any tribal elders or militia commanders. "I don't understand why the passenger vehicle was targeted," he said.
Dilawar Khan, a survivor, said he heard a huge blast, and the passengers dived to the ground: "There was a big bang and we all were lying here and there ... I was listening to people's cries but unable to see anything as dust and smoke engulfed the air. Then I found myself in the hospital with my leg and hand bandaged."