Witnesses have described scenes of utter devastation after a passenger jet exploded and ploughed into a field in north-west Iran.

All 153 passengers and 15 crew were killed when the Russian-made Caspian Airlines Tupolev jet crashed 16 minutes after take-off.

It was on its way to the Armenian capital Yerevan, and many of the passengers were Armenian or Iranians of Armenian descent.

Witnesses said the plane burst into flames before falling out of the air, and shattered into small pieces as it ploughed into farmland.

Apart from a wing there was barely any sign of the plane and only a few body parts scattered around the deep trench which the impact left in the field.

Authorities say the wreckage and corpses were totally consumed by the flames.

Among the dead were eight members of Iran's national junior judo team and two coaches, and, reportedly, the head of Georgia's diplomatic mission in Tehran.

Weeping relatives and friends gathered at Yerevan airport where a notice on a wall listed people who were on board. Doctors treated relatives for shock and heart problems.

Fina Karapetian, an Armenian in her 30s, said her sister and two nephews, 11 and 6, were on board the crashed plane.

"I heard everyone in the aircraft has died. What will I do without Armen and Vahe," she said, before fainting.

Iranian television showed footage of debris and body parts and a smoking trench with mangled pieces of metal scattered around.

Senior Iranian provincial official, Sirous Saberi, said the aircraft had had technical problems and tried to make an emergency landing.

"Unfortunately the plane caught fire in the air and it crashed," he said.

A witness said he had seen the plane's left engine on fire in the air, state broadcaster IRIB said.

But state radio said the pilot had made no mention of any technical problem in a taped conversation with a control tower.

"On board the plane there were 151 adults, 2 children and 15 crew members," Caspian Airline's representative in Yerevan Arlen Davudyan said at the airport.

"Fifteen or sixteen minutes after take-off the plane fell near the Iranian city Qazvin about 150 kilometres north of Tehran," he said, adding it was a Tu-154.

He said the cause of the crash was not clear nor had the black box been found.