The fire in the border town of Islam Qala spread to dozens of nearby fuel trucks, the officials said, and Waheed Qatali, governor of the western province of Herat, said Iranian authorities had been asked for urgent assistance to contain the blaze.
It was not immediately clear what caused the gas tanker to explode.
Ten injured people were taken to hospital and the number of casualties could rise, said Mohammad Shirzai a spokesman for Herat's health department.