"The explosion killed eight people and destroyed the truck completely," a senior security official said.
Sources said the men were travelling to a tribal meeting which was to decide the fate of one of their captured fighters, who had been blamed for an attack on an officer of the government-backed tribal police this week.
Residents said a US drone from neighbouring Afghanistan was in the air when the truck exploded, suggesting it might have been hit from the air.
The region is a known stronghold of Taliban and al-Qaeda fighters. A US missile strike in Damadola in Bajaur earlier this month killed 14 militants.