An unidentified man observes the scene of the Caspian Airlines plane crash near the village of Jannat Abad, outside the city of Qazvin, Iran, on Wednesday. (ISNA/Associated Press/Sina Shiri)

The fiery crash of a Russian-made jet that nosedived into a field after taking off from Tehran on Wednesday, killing all 168 people aboard, is being called Iran's worst air disaster in six years.

Caspian Airlines Flight 7908 was heading from Tehran to the Armenian capital of Yerevan. It was the latest in a string of deadly crashes in recent years that have highlighted Iran's difficulties in maintaining its aging fleet of planes.

The Tupolev jet's impact plowed a deep, long trench into agricultural fields outside the village of Jannat Abad, and the aircraft was blasted to bits. Flaming wreckage, body parts and personal items were strewn over a 200-metre area.

Firefighters put out blazes from the crash, but smoke smouldered from the pit for hours after as emergency workers searched for data recorders and other clues to the cause.

The plane crashed 16 minutes after takeoff near the city of Qazvin — located about 120 kilometres northwest of Tehran — at about 11:30 a.m. local time.

A rescue worker examines the scene of the crash. (ISNA/Sina Shiri/Associated Press)

There were 153 passengers and 15 crew members on board the TU-154M, said Arsen Pogosian, the deputy chairman of Armenia's civil aviation authority.

At Yerevan airport, Tina Karapetian, 45, said she had been waiting for her sister and the sister's six- and 11-year-old sons, who were due to arrive on the flight.

"What will I do without them?" she said, weeping, before she collapsed to the floor.

Press TV also reported that eight members, two coaches and an official with Iran's junior judo team were aboard the airplane. They were scheduled to train with the Armenian judo team before attending competitions in Hungary on Aug. 6.

It is not clear what brought the plane down.

Reports of mid-air blaze

Before crashing, the plane's tail was on fire as it circled in the air, said one witness.

Caspian Airlines Flight 7908 crashed just outside Qazvin on Wednesday morning. (CBC)

"Then, I saw the plane crashing nose down. It hit the ground causing a big explosion. The impact shook the ground like an earthquake. Then, plane pieces were scattered all over the agricultural fields," said Ali Akbar Hashemi, a 23-year-old who was laying gas pipes in a nearby home.

Several other witnesses also reported seeing the plane's tail on fire in the air as it circled to find a place to land, a police official told Iran's semi-official ISNA news agency.

Reports that the aircraft was on fire were "only one version" of the chain of events, said Serob Karapetian, the chief of Yerevan airport's aviation security service. He did not elaborate, but said the plane may have attempted an emergency landing.

Iran's Fars news agency reported that most of the passengers were Iranians and Armenians. A Caspian Airlines representative said some Georgian citizens were also on board.

But Pogosian contradicted those accounts, saying only six Armenian citizens and two Georgian citizens were on the flight, while the rest were likely Iranians.

It is not immediately clear if there were people of other nationalities on the plane.

The representative spoke on condition of anonymity because she was not authorized to talk to the press.

Caspian Airlines, a joint Iranian-Russian venture, primarily uses Russian-made Tupolevs.

President Mahmoud Ahmadinejad issued a statement expressing condolences and urging a swift investigation of the cause.

Iran has suffered a number of high-profile aircraft crashes in recent years.

Iran has placed some of the blame for its poor aviation safety record on sanctions imposed by the United States.

Iran has claimed that those U.S. sanctions impair operators' ability to perform maintenance and obtain spare parts. But the sanctions would likely have a more limited impact on Russian Tupolevs like the one involved in Wednesday's crash.

With files from The Associated Press