A passenger plane crashed in a mountainous region in southern Iran on Sunday, killing all 65 people on board, an airline spokesman told Iran state television.

Aseman Airlines Flight No. 3704 came down near the city of Yasuj, about 485 miles south of Tehran, the airline said. The ATR-72 — a twin-engine turboprop — was near its destination when it crashed into Mount Dena while carrying 59 passengers and six crew members, Iran Aseman Airlines spokesman Mohammad Taghi Tabatabai told state TV.

State-run IRNA news agency initially reported that 66 people were on board.

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

Rescue helicopters heading to the scene couldn’t land near the crash site because of heavy fog, Reuters reported. Emergency workers were trying to reach the area by land, the semi-official Mehr news agency said. Other weather-related issues prevented rescuers from finding the plane.

“Heavy snow is making it difficult for rescuers to find the site of the crash,” a TV reporter stated.

The plane disappeared from radar screens about 50 minutes after departing from Tehran, according to local media reports.

It’s unclear what caused the crash.

The aircraft only began flights months ago after it was “grounded” for seven years. The airline posted a photo of the plane on Instagram saying it would be up and running after repairs were done and safety checks and tests were conducted. It’s unclear what caused the aircraft to be grounded for an extended time.

Iran Aseman Airlines is a semi-private air carrier headquartered in Tehran that specializes in flights to remote airfields across the country. It also flies internationally. The Iranian Red Crescent said it has deployed personnel to the area. Authorities said they would be investigating.

The carrier, owned by Iran's civil service pension foundation, has a fleet of 29 aircraft, including six ATR aircraft, according to FlightRadar24, a plane-tracking website. It is Iran's third-largest airline by fleet size, behind state carrier Iran Air and Mahan Air.

European airplane manufacturer ATR, a Toulouse, France-based partnership of Airbus and Italy's Leonardo S.p.A., said it had no immediate information about the crash. The manufacturer specializes in regional turboprop aircraft of 90 seats or less.

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. U.S. politicians have expressed concern about the airplane sales to Iran. President Trump remains skeptical of the atomic accord overall and has refused to re-certify it, putting the deal in question.

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

In April 2017, ATR sealed a $536 million sale with Iran Air for at least 20 aircraft. Chicago-based Boeing also signed a $3 billion deal that month to sell 30 737 MAX aircraft to Aseman Airlines.

The Associated Press contributed to this report.