Eyewitnesses claimed that diesel was leaking out of the oil tanker after the crash and footage showed the bus in flames with smoke rising from the vehicle.
The bus was taking passengers from Tehran to the city of Kerman and the collision occurred on a road linking the cities of Kashan and Natanz.
At least 20 people were injured in the crash and were taken to local hospitals for treatment.
An unnamed survivor told state television that the crash happened close to midnight.
"The flames started at the front of the bus as diesel from the tanker poured in," he said, adding that he managed to escape through a bus window.
The exact cause of the crash remains unknown.
Many Iranians are travelling this week as the four-day major Shiite Muslim holiday of Ashura gets underway.
With some 17,000 motorists killed annually, Iran has one of the world's worst traffic safety records, attributed mainly to widespread disregard for traffic laws, unsafe vehicles and inadequate emergency services.
The Independent
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