The truck, hired by Bangladesh Red Crescent Society and the International Red Cross Committee to carry relief materials for Rohingya refugees, skidded off the road after its driver lost control near the Chhakdala border post, some 50 kilometres from Cox's Bazar.
"Six of them died instantly as the accident occurred this morning when the truck skidded off the road," chairman of the village council told reporters in Badbarban.
Nine people died and five others sustained injures in the accident, police said.
The crash victims were all workers who were hired to unload and carry on foot the relief material to remote refugee camps as no vehicle could reach the rugged site, she said.
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A Bangladesh Red Crescent official said the truck was carrying rice, puffed rice, drinking water, sugar and other food items.
Bangladesh Red Crescent Society and the International Red Cross Committee hired the truck to carry the materials, the official added.
Bangladesh has seen a massive influx of Rohingya people fleeing brutal violence in Myanmar in the last two weeks.
The most recent influx started after an attack on August 25 on police posts in Myanmar which killed 12 security forces and more than 50 militants.
Aid agencies have started a huge relief operation in the border districts of Cox's Bazar and Bandarban.
They say they have been surprised by the number of the influx from Myanmar.