The huge explosion took place in the early hours of Saturday at a gas pressure boosting station near Shul village and some 35 kilometers (21 miles) from the port city in Bushehr province, IRNA quoted Genaveh Governor Ali Paknejad as saying.
The blaze is so huge that firefighters have not managed to reach it yet, he added, warning people not to come close to the scene for safety reasons.
Witnesses said the raging blaze could be clearly seen from a distance of 30 kilometers (18.5 miles).
Abdolhamid Khedri, Bushehr province's representative at the Iranian parliament, ruled out sabotage as the cause of the incident.
IRNA reported that immediately after the blast, the pipeline was disconnected from the rest of the system to limit the scale of the damage.
