Iranian rescue teams battled severe weather Monday as they searched for 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 EP3704 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.
"Teams are searching by foot and so far they have not found anything."
Officials said hundreds of mountaineers, dogs and drones were operating around the Dena mountain at altitudes as high as 4,500 metres (15,000 feet).
The ATR-72 twin-engine plane, in service for 25 years, left the capital's Mehrabad airport at around 8:00 am (0430 GMT) Sunday and was heading towards the city of Yasuj, some 500 kilometres (300 miles) to the south.
Several relatives of passengers have travelled to the Dena mountain area where the plane is thought to have come down, local media said.
A team of crash investigators from French air safety agency BEA was set to arrive in Iran later on Monday.
"Three investigators and our technical advisers will go to the site," a BEA spokesman told AFP.
- Aseman blacklisted -
Aseman Airlines was blacklisted by the European Commission in December 2016 -- one of only three airlines barred over safety concerns.
The other 190 airlines banned by Europe were blacklisted due to broader concerns over safety 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.
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 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.
However, the sale could be scuppered if US President Donald Trump chooses to reimpose sanctions in the coming months, as he has threatened to do.