All 66 people on board an Iranian passenger plane were feared dead on Sunday after it crashed into the country's Zagros mountains, with emergency services struggling to locate the wreckage in blizzard conditions.
Aseman Airlines flight EP3704 disappeared from radar around 45 minutes after takeoff from Tehran's Mehrabad airport, the airline's public relations chief Mohammad Tabatabai told state broadcaster IRIB.
It was the third disaster to strike Iran in recent months, after an earthquake that killed at least 620 people in Kermanshah in November and 30 Iranian sailors were lost in an oil tanker collision off China's coast last month.
The ATR-72 twin-engine plane, in service for 25 years, left the capital around 0800 (0430 GMT) and was heading towards the city of Yasuj, some 500 kilometres (300 miles) to the south.
After conflicting reports on fatalities and the location of the crash, officials said rescue teams were still not able to find the wreckage.
"We still have no access to the spot of the crash and therefore we cannot accurately and definitely confirm the death of all passengers," Tabatabai told the ISNA news agency.
He said the plane was carrying 60 passengers, including one child, as well as six crew.
Jalal Pooranfar, regional head for Iran's emergency services, told the ISNA news agency rescue and relief teams had been sent to the possible area of the crash.
"But the helicopter could not continue its path due to snow and blizzard," he said.
Seyed Noor Mohammad Mousavi, head of the local Red Crescent office, told the IRNA news agency a drone had been dispatched to help find the wreckage.
A total of 120 people from 30 different emergency teams were sent to help with the search, another Red Crescent official said.
Supreme leader Ayatollah Ali Khamenei sent a message of condolence, saying the news had "left our hearts overwhelmed with sadness and sorrow", according to state television.
A man who missed the doomed flight told reporters of his mixed emotions.
"God has been really kind to me but I am so sad from the bottom of my heart for all those dear ones who lost their lives," the unnamed man told the Tabnak news agency, which showed a picture of his unused ticket.
Decades of international isolation have left Iran's airlines with ageing fleets of passenger planes which they have struggled to maintain and modernise.
Aseman's fleet includes at least three ATR-72s that date back to the early 1990s, according to the IRNA news agency.
A spokesman for ATR, which is part-owned by Europe's Airbus, told AFP the company was "researching the details" of Sunday's crash.
President Hassan Rouhani ordered the transport ministry to set up a crisis group to investigate the crash and coordinate rescue efforts, ISNA reported.
Aseman's three Boeing 727-200s are almost as old as the country's 1979 Islamic revolution, having made their first flights the following year.
Iran has suffered multiple aviation disasters, most recently in 2014 when a Sepahan Airlines plane crashed killing 39 people just after take-off from Tehran, narrowly avoiding many more deaths when it plummeted near a busy market.
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.
Israeli Prime Minister Benjamin Netanyahu offered his condolences over Sunday's crash, just moments after he launched a blistering attack on Tehran's government.
"I take this opportunity to send condolences to the families of the 66 civilians that lost their lives," Netanyahu said at the Munich Security Conference.
"We have no quarrel with the people of Iran, only with the regime that torments them," he added.
The US Treasury Department, which must approve aviation sales to Iran, has done so for 80 Boeing jets destined for national carrier Iran Air as well as 100 Airbus planes for Iran Air.
The first few Airbus jets have already arrived in Tehran.