Petroleos Mexicanos, or Pemex, said in a statement that four charred bodies and a burnt-out vehicle were found at the scene of the explosion near the town of El Mango, in the municipality of Tierra Blanca.
It added that they were not Pemex employees and noted that "fuel theft represents a grave risk for people, communities and the environment."
Pemex said it turned off pressure in the duct, put out the fire and was working to close the tap.
Mexico's treasury secretary said Tuesday that fuel theft costs the country $780 million to $1 billion a year.
Clashes between the military and suspected fuel thieves killed 10 people recently in the central state of Puebla. Video later surfaced showing a soldier apparently executing a wounded suspect.