"The plane, which was initially headed west to leave the airport zone, turned right following a problem and was headed back to the airport at the moment of the crash," the Iranian Civil Aviation Organisation said on its website late Wednesday.
"The plane disappeared from radar screens the moment it reached 8,000 feet (2,400 metres). The pilot sent no radio message about the unusual circumstances.
"According to eyewitnesses, a fire was seen on board the plane which grew in intensity," the organisation added, reporting the first findings of its investigation into Wednesday's crash.
The organisation said it had questioned witnesses both on the ground and on board a second aircraft which was flying above the Ukrainian Boeing 737 as the disaster unfolded.