President Claudia Sheinbaum told reporters on Thursday such “tragic” accidents must not be allowed to happen again.
Images distributed on television and social networks showed the moment of the powerful explosion.
People could be seen with what appear to be serious burns, while others fled fast-spreading flames that were later brought under control by firefighters.
About 28 other vehicles were damaged in the explosion, the cause of which is not yet known.
Some victims were evacuated by helicopter as part of an emergency response that involved hundreds of paramedics and soldiers.
Images showed the burning truck, which was transporting nearly 50,000 litres of gas, overturned on the road.
Preliminary information indicated the trailer’s operating permit was not in order, officials said. A federal safety agency said the vehicle’s owner had not submitted the required insurance documents.
Iztapalapa is home to 1.8 million of Mexico City’s 10 million-odd inhabitants, making it one of the most populated districts in the country.
Mexico is no stranger to disasters linked to fuel trucks and hydrocarbon infrastructure.
The worst occurred in January 2019, when a fire and subsequent explosion on a pipeline being looted killed 137 people in the town of Tlahuelilpan in the central state of Hidalgo.
– Agence France-Presse