TEHRAN - Iranian rescue teams were forced to suspend their hunt for a missing passenger plane for a second night on Monday as severe weather and hazardous mountain conditions thwarted search efforts.
Aseman Airlines flight EP3704 disappeared on Sunday morning in the Zagros mountains with 66 people on board.
Officials said 60 helicopter sorties had flown on Monday to no avail.
"The exact spot of the plane crash was not found, and given the darkness, heavy snowfall and fog in some regions, the aerial search operation was stopped and will be resumed tomorrow," Esmaeil Najjar, head of Iran's Crisis Management Organisation, told the ISNA news agency.
More than 120 mountaineers are deployed, he said, and "will stay in safe places in the mountainous area before resuming search operations tomorrow".
The search has been focused on the 14,465-foot (4,409-metre) Dena mountain, which is popular with Iranians training for climbs 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 (300 miles) 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 sample kits 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.
Both Russia and France have provided satellite images but nothing has yet been found in them, the Civil Aviation Organisation told IRIB.
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 blacklisted -
Aseman Airlines was blacklisted by the European Commission in December 2016.
It was one of only three airlines barred over safety concerns -- another 190 were blacklisted due to broader concerns over oversight in their respective countries.
Iran has complained that sanctions imposed by the United States have jeopardised the safety of its airlines, making it difficult to maintain and modernise ageing fleets.
Aseman was forced to ground many of its planes at the height of sanctions due to difficulties in obtaining spares, Taghvaee said.
In a working paper presented to the United Nations' International Civil Aviation Organization (ICAO) in 2013, Iran said US sanctions were barring "the acquisition of parts, services and support essential to aviation safety".
Iran has suffered multiple aviation disasters, most recently in 2014 when 39 people were killed as a Sepahan Airlines plane crashed just after take-off from Tehran, narrowly avoiding many more deaths when it plummeted near a busy market.
But figures from the Flight Safety Foundation, a US-based NGO, suggest Iran is nonetheless above-average in implementing ICAO safety standards.
Lifting sanctions on aviation purchases was a key clause in the nuclear deal that Iran signed with world powers in 2015.
Following the deal, Aseman Airlines finalised an agreement to buy 30 Boeing 737 MAX jets for $3 billion (2.4 billion euros) last June, with an option to buy 30 more.
The sale could still be scuppered if US President Donald Trump chooses to reimpose sanctions in the coming months, as he has threatened to do.