A Ukrainian airliner that crashed outside of Tehran was on fire and trying to turn back but its crew never made a radio call for help, Iranian investigators said.

The Ukrainian International Airlines Boeing 737-800, flying to Kyiv and carrying mostly Iranians and Iranian-Canadians, crashed shortly after taking off on Wednesday from Tehran's Imam Khomeini airport, killing all 176 on board.

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Witnesses, including the crew of another passing flight, described seeing the plane engulfed in flames before crashing, the investigators' report said. The crash caused an enormous explosion when the plane hit the ground, likely because the aircraft had been fully loaded with fuel for the four-hour flight.

"The plane, which was initially headed west to leave the airport zone, turned right following a problem and was headed back to the airport at the moment of the crash," the Iranian Civil Aviation Organisation said on its website.

"The plane disappeared from radar screens the moment it reached 8,000 feet [2,400 metres]. The pilot sent no radio message about the unusual circumstances."

Ukraine outlined four potential scenarios on Thursday to explain the deadly crash, including a missile strike and terrorism.

Kyiv said its investigators wanted to search the site of Wednesday's crash southwest of Tehran for possible debris of a Russian-made missile used by Iran's military.

'Refrain from speculation'

Ukrainian President Volodymyr Zelenskyy, meanwhile, said crash investigators from his country had arrived in Iran to assist with the probe.

"Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash," Zelenskyy said. "We will surely find out the truth." He also declared Thursday a day of national mourning.

In a televised statement, Zelenskyy earlier asked people to refrain from speculation, conspiracy theories and hasty evaluations regarding the crash.

Both flight recorders - known as "black boxes" - from the three-year-old jet were found and are being investigated. Iranian Transport Minister Mohammed Eslami said they would be handed over to Ukraine.

The country that manufactures the aircraft is also supposed to have representatives involved in such a probe.

In theory, this means that the US National Transportation Safety Board (NTSB), the body charged with investigating air accidents, would be involved. The aeroplane's manufacturer, Boeing, is based in the US, and investigators would likely rely on experts from the manufacturer.

However, the head of Iran's Civil Aviation Organisation, Ali Abedzadeh, said while Ukrainians were free to participate in the investigation into the crash, "we will not give the black boxes to the manufacturer [Boeing] and the Americans", according to Iran's Mehr news agency.

A Canadian security source told Reuters news agency there was evidence one of the jet's engines had overheated.

The crash happened hours after Iran launched missile attacks on US-led forces in Iraq, in retaliation for the US assassination of a top Iranian commander, leading some to speculate that the plane may have been hit.

The initial assessment of Western intelligence agencies was that the plane suffered a technical malfunction and had not been brought down by a missile, five security sources - three Americans, one European and the Canadian - who asked not to be named, told Reuters.

Bodies and body parts recovered from the site of the crash have been taken to the coroner's office for identification, the investigators' initial report said.

Smouldering debris, including shoes and clothes, was strewn across the field where the plane crashed on Wednesday, as rescue workers in face masks laid out dozens of body bags.

On board were 146 Iranians, 10 Afghans, 11 Ukrainians, five Canadians and four Swedes, the report said but noted some may have held citizenship of other countries.

Ukrainian authorities have said those on board included 82 Iranians, 63 Canadians, and 11 Ukrainians.

'Many questions'

Canadian Prime Minister Justin Trudeau said 138 of the passengers were bound for Canada. The flight also included a family of four as well as newlyweds. The manifest listed several teenagers and children, some as young as one or two.

The crash ranked among the worst losses of life for Canadians in an aviation disaster. The flag over Parliament in Ottawa was lowered to half-staff and Trudeau vowed to get to the bottom of the disaster.

"Know that all Canadians are grieving with you," he said, addressing the victims' families.

Canadian Foreign Minister Francois-Philippe Champagne called his Iranian counterpart to stress the need for Canadian officials "to be quickly granted access to Iran to provide consular services, help with identification of the deceased and take part in the investigation of the crash", a Canadian statement said.

"Canada and Canadians have many questions which will need to be answered."