According to police and rescue officials, the bus was filling gas at a CNG station when the blast occurred.
Around 10 people were on board the bus.
The CNG station has been cordoned off while the bus is being escorted to the police station for investigations.
CNG cylinders in vehicles have been the cause of several explosions in the past, taking scores of lives across the country.
The last cylinder blast in Karachi was reported in September which resulted in the death of a 10-year-old boy and injured six people.
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