An Iranian commercial airplane brought back into service only months ago after being grounded for seven years crashed on Sunday in a foggy, mountainous region of southern Iran, and officials said they feared all 65 people on board were killed.

The crash of the Aseman Airlines ATR-72 marks yet another fatal aviation disaster for Iran, which for years was barred from buying airplane parts for needed maintenance due to Western sanctions over its contested nuclear program.

Its nuclear accord with world powers allows it now to get those parts and the country has made deals worth tens of billions of dollars for new aircraft. However, U.S. President Donald Trump's refusal to recertify the deal has injected uncertainty into those sales, while Iranians still fly in aging aircraft.

The ATR-72, a twin-engine turboprop used for short-distance regional flying, went down near its destination of the southern Iranian city of Yasuj, some 780 kilometres south of the Iranian capital, Tehran, where it took off.

After searching the area, we learned that unfortunately ... our dear passengers had lost their lives. - Mohammad Taghi Tabatabai, Aseman Airlines spokesperson

It wasn't immediately clear what caused the crash, although weather was severe in the area. Dense fog, high winds and heavy snow in the Zagros Mountains made it impossible for rescue crews in helicopters to reach the site Sunday, state television reported.

Aseman Airlines spokesperson Mohammad Taghi Tabatabai told state TV that all on board Flight No. EP3704 were killed. Those on board included 59 passengers and six crew members, the state-run IRNA news agency reported Sunday night, lowering the death to 65 from an initially reported 66.

"After searching the area, we learned that unfortunately ... our dear passengers had lost their lives," Tabatabai said.

Iranian Red Crescent responding

Both Supreme Leader Ayatollah Ali Khamenei and President Hassan Rouhani offered their condolences.

Tabatabai said the plane crashed into Mount Dena, which is about 4,400 metres tall. The plane's last signal, at 12:35 a.m. ET, showed it at 5,173 metres and descending, according to airplane-tracking website FlightRadar24. The pilot was in contact with the tower 22 kilometres from the airport, state TV said.

One previous passenger on the route posted a video Sunday showing that the flight typically comes in just over the mountain peaks. Aeronautical charts for the airport warn pilots to keep an altitude of 4,500 metres in the area. The airport itself is at nearly 1,800 metres.

The flight was en route to the southern city of Yasuj after departing from Tehran. (Google)

The Iranian Red Crescent said it has deployed staff to the area. Locals described hearing the crash, although no one had found the crash site yet, according to state TV.

Aseman Airlines, owned by Iran's civil service pension foundation, is a semi-private air carrier headquartered in Tehran that specializes in flights to remote airfields across the country. It also flies internationally.

Aseman Airlines is Iran's third-largest airline by fleet size, behind state carrier Iran Air and Mahan Air. However, it is banned from flying in the European Union over safety concerns.

The carrier has a fleet of 29 aircraft, including six ATR aircraft, according to FlightRadar24. The ATR-72 that crashed Sunday, with the tail number EP-ATS, had been built in 1993, Aseman Airlines CEO Ali Abedzadeh told state TV.

On Instagram, Aseman Airlines highlighted the doomed aircraft in October, saying it had been "grounded" for seven years but would be "repaired and will be operational after checking and testing."

Reason for grounding unclear

It wasn't clear what led to the grounding, though Iran only recently regained access to the airplane parts market after the nuclear deal.

European airplane manufacturer ATR, a Toulouse, France-based partnership of Airbus and Italy's Leonardo SpA., said it had no immediate information about the crash.

Aseman Airlines has suffered other major crashes with fatalities. In October 1994, a Fokker F-28 1000 commuter plane operated by the airline crashed near Natanz, 290 kilometres south of Tehran, killing 66 people on board. An Aseman Airlines chartered flight in August 2008, flown by an Itek Air Boeing 737, crashed in Kyrgyzstan, killing 74 people.

Under decades of international sanctions, Iran's commercial passenger aircraft fleet has aged, with air accidents occurring regularly in recent years.

Following the 2015 landmark nuclear deal with world powers, Iran signed deals with both Airbus and Boeing to buy scores of passenger planes worth tens of billions of dollars.

In April 2017, ATR sealed a $673 million Cdn sale with Iran Air for at least 20 aircraft. Chicago-based Boeing also signed a $3.77 billion Cdn deal that month to sell 30 737 MAX aircraft to Aseman Airlines.

Home to 80 million people, Iran represents one of the world's last untapped aviation markets. However, Western analysts are skeptical that there is demand for so many jets or available financing for deals worth billions of dollars.