THE pilot of a Ukranian airliner which crashed in Tehran did not make a call for radio help and was trying to turn back to the airport when the plane plunged from the sky, according to Iranian investigators.

Iran is refusing to hand over the black box recorders of the Boeing 737-800, which burst into flames in mid-air before crashing yesterday - killing all 176 people on board.

29 A section of the Ukraine International Airlines plane lies in a field outside Tehran

29 The plane's black box flight recorder was pictured on Iranian TV

29 Rescue workers walk past rows of body bags at the scene of the plane tragedy Credit: AP:Associated Press

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The tragedy came just hours after Tehran fired at least 15 ballistic missiles at two US bases in Iraq, sparking fears it could have been shot down by mistake by jittery Iranian air defences.

Iran has launched its own investigation and said today the pilot was trying to turn back to the airport when the Kiev-bound flight crashed three minutes after take off.

But Iran's Civil Aviation Organization offered no immediate explanation for the disaster.

Officials had previously hinted the plane may have suffered engine failure.

Three American sources, along with one European and one Canadian, told Reuters the initial assessment of Western intelligence agencies was that the plane had suffered a technical malfunction.

There was evidence one of the Ukraine International Airlines jet's engines had overheated, the Canadian source said.

Horrifying footage posted online shows the plane engulfed in flames as it went down.

Three Britons and 63 Canadians were among the 168 passengers and nine crew who on board the plane.

Canadian Prime Minister Justin Trudeau vowed to “ensure that this crash is thoroughly investigated, and that Canadians' questions are answered”.

The plane’s black box flight recorders have been found, with one Iranian official saying they were damaged but their data could still be retrieved.

BALL OF FLAMES

However, Iran is refusing to hand them over to international authorities.

Iranian media said the Boeing 737 came down after "technical problems" and quoted an aviation chief who said no emergency had been declared.

Qassem Biniaz, a spokesman for Iran's Road and Transportation authority, claimed one of its engines caught fire, causing the pilot to lose control.

But the aircraft was new in 2016 and was last checked just two days ago, the Ukrainian foreign ministry said.

Engine maker CFM slammed claims the crash was caused by engine failure, saying it was too early to speculate on what was behind the tragedy.

Ukraine's Tehran embassy initially blamed engine failure but later removed the statement, saying it could not officially comment on the cause until after a probe.

It also abruptly removed a line from its statement excluding a terrorist attack or missile strike.

29 Two victims of the crash have been named as Alvand and Sadeghi Borghei, who lived in Toronto Credit: Reuters

The plane crashed shortly after take-off at Tehran Airport

29 Footage appears to show the jet in flames before hitting the ground

29 The plane was filmed hitting the ground in a ball of flames Credit: Twitter

29 Harrowing images showed victims placed in body bags at the scene Credit: AFP or licensors

29 A child's book lies among the wreckage strewn across the ground Credit: Getty - Contributor

29 The scorched crash site is seen form air air after the plane slammed into the ground Credit: AFP

29 Smoke rises after the fireball killed all 176 people on board Credit: MIZAN

29 Rescue teams examine what appears to be the wheel of the airliner in the wreckage Credit: AFP or licensors

29 Emergency crews were rushed to the scene but all on board were found to have perished Credit: AFP

29 Wreckage could be seen strewn across the ground in a field near the airport Credit: EPA

29 Emergency crews sift through the wreckage this morning Credit: AFP or licensors

29 The possessions of victims were collected near the crash site Credit: EPA

Among the dead were Alvand Sadeghi, 30, a talented pianist who played for guests at his 2018 wedding, to his wife Negar Borghei, who was also killed.

Also killed were newly wed couple Arash Pourzarabi, 26, and Pouneh Gourji, 25, who had travelled to Iran to get married.

They were on the plane with four members of their wedding party and another 24 Iranian-Canadians from Edmonton.

The flight was a popular transit route for Canadians travelling to Iran, in the absence of direct flights, and carried many students and academics heading home from the holidays.

Flags flew at half-mast across Canada, including at Parliament in Ottawa, and vigils were scheduled in several Canadian cities.

Flight data shows the plane reached an altitude of 2.4 km and then suddenly disappeared.

The three-year-old plane stopped sending data almost immediately after it took off, according to website FlightRadar24.

Rescuers rushed to the scene of the crash but could not get near the site because of the intensity of the flames.

29 A woman fights back tears as she waits for news at Kiev Airport Credit: Reuters

29 Tearful relatives were seen at the airport in Ukraine where the plane was due to land Credit: Reuters

29 Flight trackers show the plane crashed near Parand just moments after take-off Credit: flightradar24.com

29 The plane disappeared from flight radars around three minutes after take-off

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Images show flaming wreckage strewn across the ground in the immediate aftermath of the crash.

And other harrowing pictures show victims being lined up in body bags at the scene and their possessions collected in a field nearby.

Some charred remains of the aircraft had fallen in or close to a residential area.

Rescuers at the scene moments after the crash in Tehran

29 Burning wreckage is seen strewn across the ground ion the immediate aftermath Credit: AFP or licensors

29 Rescuers could not get near the crash site at first because the fire was too intense Credit: AFP or licensors

29 Iranian state media claimed the plane caught fire after suffering engine problems Credit: IRCS

Authorities said 'technical problems' caused the deadly crash

29 Small charred parts of the plane were found on the ground Credit: AP:Associated Press