The incident took place in the southern Jamshoro district, not far from Karachi.
"The tire of the van burst and then it fell into a ditch. This caused the explosion in its cylinder," Jamshoro district police head Naeem Shaikh was quoted as saying by the Pakistan Herald.
No fire extinguishing facilities were located near the place of the accident. Attempts by the motorway police and locals to contain the fire were unsuccessful, according to the newspaper.
Only those sitting near the van's doors could get out as it caught fire, Shaikh said. Injured passengers, including two children, were rushed to hospital.
Ten bodies were recovered from the van, all beyond recognition, and transported to Karachi for DNA testing. The driver is believed to be among the dead.