Iran's transport ministry says the loss of an ATR 72 turboprop operated by Iran Aseman Airlines has resulted in 66 fatalities.
The ministry has established an operations centre to handle the aftermath of the crash.
It states that the aircraft, operating between Tehran and Yasouj, came down near Semirom in west-central Iran.
Fatalities included 60 passengers and six crew members, it adds.
Iran's president has ordered the establishment of a commission to investigate the accident.
The Iranian Civil Aviation Organisation states that the crash site is mountainous and experiencing poor weather conditions.
Flight EP3704 had been due to depart at around 08:00 on 18 February.
Iran Aseman Airlines has around half a dozen ATR 72s in its fleet. The identity of the airframe involved has not been confirmed.