The blaze started in a bitumen storage tank at the refinery in the Jannatabad industrial zone and was briefly contained before reigniting, said Hossein Derakhshan, head of the Semnan Red Crescent.
He said firefighting and rescue teams from Eyvanaki, Garmsar, and nearby towns were deployed to the site, and the fire was expected to be fully extinguished within hours.
The cause of the incident is under investigation.