The comments corrected an earlier report of 80 dead, which had come from state-run television.
The plane, carrying 148 people, slid off the runway, "then its left wing hit the ground and caught fire", Nourollah Rezai Niaraki, chairman of Iran's Civil Aviation Organisation, said in an interview on the television.
Niaraki said it was not immediately known why the plane skidded off the runway. Earlier, the television said a tyre on the aircraft exploded while landing, but that could not be confirmed.
Television pictures showed a broken-up plane lying on the side of the runway with parts of its fuselage charred. The cockpit appeared to be largely unaffected by the fire, as did much of the rear portion of the plane.
Firefighters were shown extinguishing fires in parts of the smouldering wreck and clambering over other areas of the fuselage, carrying out corpses covered in blankets.
An Iranairtour official confirmed the incident involved one of its planes but gave no details.
Pilgrims flock to Mashhad throughout the year to visit the tomb of Imam Reza, the eighth Muslim Shia imam. It was not clear if any of those on board the Iranairtour flight were making the pilgrimage.
TV showed a broken-up plane
lying on the side of a runway
Bandar Abbas, where the plane began its journey, is the Islamic Republic's main port. Nearby the port is the popular Iranian holiday destination of Qeshm island.
Air safety experts say Iran has a poor safety record with a string of crashes in recent decades, many involving Russian-made aircraft.
US sanctions on the Islamic state have prevented it from buying new aircraft or spares from the West, forcing it to supplement its ageing fleet of Boeing and Airbus planes with aircraft from the former Soviet Union.
The last major plane disaster in Iran involved a military plane which crashed in January, killing at least 11 people. Another military plane hit a tower block in Tehran in December, killing 94 people on board and at least 22 people on the ground.