The incident, reportedly caused after the explosion of three gas canisters, took place in one of the ground floors of the Sina At''har Medical Centre in northern Tehran late Tuesday, Press TV said in a report.
Several emergency response units, including ambulances and firefighters, were seen deployed on site as smoke billowed from the building.
A spokesman for Tehran''s Fire Department told state TV that the explosion occurred as gas canisters caught fire in the centre''s basement.
Some of the victims "were in upper floors in operation rooms, who were either patients being operated on or those with them", Press TV quoted the spokesman as saying.
"They unfortunately lost their lives due to the heat and thick smoke."
The Department added that the blaze was extinguished in less than two hours.