Iranian rescue teams battled severe weather on Monday as they searched for the wreckage of a passenger plane that disappeared high in the Zagros mountains the previous day with 66 people on board.
Several helicopters that had deployed at dawn to hunt for Aseman Airlines flight were forced to return to base, officials said.
"Unfortunately due to strong winds and fog reducing visibility, it was not possible for helicopters to continue their search," a Red Crescent official told the ISNA news agency.
Officials said hundreds of mountaineers, supported by dogs and drones, were operating around the 4,409-metre Dena mountain, which is popular with Iranians seeking to prepare for climbing in the Himalayas.
The ATR-72 twin-engine plane, in service since 1993, flew early Sunday from Mehrabad airport towards the city of Yasuj, some 500 kilometres to the south.
The plane's emergency locator transmitter was reportedly not functioning, helping to explain the difficulty in finding the wreckage.
Families of the passengers had travelled to the area and were giving DNA samples for later identification of victims, the IRNA news agency reported.
A team of crash investigators from French air safety agency BEA was set to arrive in Iran later on Monday.
An ATR-72 crashed in similar icy conditions in Indiana in the United States in 1994, leading some operators to avoid cold weather conditions.
"It is a very safe aircraft but... operators decided not to use it in cold mountain areas in the US," said Iranian aviation expert Babak Taghvaee.
"Even newer versions of this aircraft are not good for such cold places and it would be better not to use it for this route and especially with such bad weather and visibility," he said.
Aseman Airlines was blacklisted by the European Commission in December 2016.
It was one of only three airlines barred over safety concerns -- the other 190 were blacklisted due to broader concerns over oversight in their respective countries.
Airlines spokesman Mohammad Taghi Tabatabai told state TV that all on board Flight EP3704 were killed.
The crash was yet another aviation disaster for Iran, which for years was barred from buying necessary airplane parts due to western sanctions over its controversial nuclear programme.
The site is reportedly at a height of 3,500 meters.
Following the landmark 2015 nuclear accord with world powers that lifted international sanctions on Tehran in exchange for curbs on its nuclear enrichment program, Iran is allowed to purchase airplanes and airplane parts and has made deals worth tens of billions of dollars for new aircraft.
However, President Donald Trump's refusal to recertify the deal has injected uncertainty into those sales.
The ATR-72 went down near its destination of the southern city of Yasuj, some 780 kilometers south of the capital, Tehran, from where it took off.
It wasn't immediately clear what caused the crash, although weather was severe. Dense fog, high winds and heavy snow in the Zagros Mountains made it impossible for rescue crews in helicopters to reach the site in the immediate aftermath, state television reported.
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