0137 GMT February 19, 2020
The accident happened early hours of the morning when the driver of the bus lost control and the vehicle plunged into a 50-meter-deep valley.
Firefighters and rescue teams were informed of the crash at 2:10 am local time on Tuesday, spokesman for the Tehran Fire Department, Jalal Maleki, told Tasnim News Agency, saying the bus was carrying 38 people.
The bus had veered off the road for unknown reasons, a report by IRNA said.
However Tehran Times daily quoted the police as saying that initial investigations show mechanical failure was the main cause of the accident.
According to the figures released by World Atlas in September 2016, Iran is ranked 4th in the list of countries with the highest road traffic death rates.
The estimated road traffic death rate per 100,000 people in Iran was 34.1.