Local administrator Shabbir Mengal said the bus was travelling to Baluchistan from the port city of Karachi when the collision happened near the town of Hub.
DNA tests will be needed to identify most of the victims, whose bodies suffered burns.
After being taken to hospital, 14 died later died from injuries sustained in the blast. At least two more remained in critical condition.
Investigators later established that flames engulfed the bus after fuel, smuggled from Iran, which borders Balushistan exploded. It was being transported in plastic containers.
Road accidents are common in Pakistan, where traffic laws are weakly enforced.
In March 2014, at least 35 died in a crash involving two buses, again smuggling Iranian petrol, and two lorries in Baluchistan province, near Gadani.
At least 148 people were killed after rushing to collect spilled fuel when an oil tanker overturned on a highway about 60 miles southwest of Multan in June 2017.
At least 23 people died after the minibus plunged 100 metres into a river, after the driver veered off a mountain road after losing control of the vehicle in Nausehri in September 2016.
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