The Ukrainian plane that crashed in Iran, killing all 176 people on board, was on fire and attempting to turn back to the airport when it came down, according to a report by Iranian investigators.

The report by Iran’s Civil Aviation Organisation (CAO) has said the crew of the jetliner never made a radio call for help and suggested a sudden emergency struck the Boeing 737 when it crashed just moments after taking off from Imam Khomeini International Airport in Tehran.

However, no immediate explanation has been given by investigators for the disaster.

Although the report confirmed both of the so-called “black boxes” which contain data and cockpit communications from the plane had been recovered, investigators said they were damaged and some parts of their memory had been lost.

Eyewitnesses have described seeing the plane engulfed in flames before crashing, according to the report.

Iran plane crash: Ukraine Boeing 737 comes down near Tehran Show all 18 1 /18 Iran plane crash: Ukraine Boeing 737 comes down near Tehran Iran plane crash: Ukraine Boeing 737 comes down near Tehran People stand near the wreckage after a Ukrainian plane carrying 176 passengers crashed near Imam Khomeini airport in Tehran ISNA/AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran People and rescue teams are pictured amid bodies and debris All 176 people on board a Ukrainian passenger plane were killed when it crashed shortly after taking off, Iranian state media reported ISNA/AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran One of the engines State news agency IRNA said 167 passengers and nine crew members were on board the aircraft operated by Ukraine International Airlines Iran Press via Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue teams work at the scene AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran Passengers' belongings West Asia News Agency via Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Members of the International Red Crescent collect bodies of victims EPA Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue teams work amidst debris AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran A relative of a victim reacts at Boryspil International Airport, outside Kiev Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue workers search the scene where a Ukrainian plane crashed in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane with more than 170 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran TOPSHOT - People and rescue teams are pictured amid the wreckage after a Ukrainian plane carrying 176 passengers crashed near Imam Khomeini airport in the Iranian capital Tehran early in the morning on January 8, 2020, killing everyone on board. - The Boeing 737 had left Tehran's international airport bound for Kiev, semi-official news agency ISNA said, adding that 10 ambulances were sent to the crash site. (Photo by Rouhollah VAHDATI / ISNA / AFP) (Photo by ROUHOLLAH VAHDATI/ISNA/AFP via Getty Images) ROUHOLLAH VAHDATI ISNA/AFP via Getty Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue workers carry the body of a victim of an Ukrainian plane crash in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran Part of the wreckage Iran Press via Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Debris of a plane belonging to Ukraine International Airlines, that crashed after taking off from Iran's Imam Khomeini airport, is seen on the outskirts of Tehran, Iran January 8, 2020. Nazanin Tabatabaee/WANA (West Asia News Agency) via REUTERS ATTENTION EDITORS - THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY WANA NEWS AGENCY West Asia News Agency via Reuters Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue workers search the scene where an Ukrainian plane crashed in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran Rescue team work at the scene where an Ukrainian plane crashed in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran CORRECTS YEAR - Debris is seen from a plane crash on the outskirts of Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying at least 170 people crashed on Wednesday shortly after takeoff from Tehranâ€™s main airport, killing all onboard, state TV reported. (AP Photos/Mohammad Nasiri) Mohammad Nasiri AP Iran plane crash: Ukraine Boeing 737 comes down near Tehran Officials inspect the wreckage EPA Iran plane crash: Ukraine Boeing 737 comes down near Tehran Debris is seen from an Ukrainian plane which crashed as rescue workers search the scene in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all onboard. (AP Photo/Ebrahim Noroozi) Ebrahim Noroozi AP

Investigators have also initially ruled out laser or electromagnetic interference as a cause for the crash.

The findings follow suggestions by some aviation experts that the crash may have been a “shootdown event”, similar to the MH17 crash in 2014.

The OPS group, an aviation risk-monitoring group, has recommended taking “the starting assumption to be that this was a shootdown event” until evidence to the contrary emerges.

Iran has said it will not work with the US in its investigation of the crash.

“We will not give the black box to the manufacturer and the Americans,” Ali Abedzadeh, head of the CAO, told Iran’s Mehr news agency.

“This accident will be investigated by Iran's aviation organisation but the Ukrainians can also be present.”

The US National Transportation Safety Board would usually have a role in international investigations involving US-made Boeings, but Iran has the right to lead the investigation under global aviation rules.

Boeing has extended condolences to the victims' families and said it stands ready to assist.

All of the plane’s 167 passengers and nine crew members were killed in the crash, which caused a massive explosion as the aircraft hit the ground, likely due to it being fully loaded with fuel for the flight to Kyiv, Ukraine.

The victims of the disaster came from several countries, including 82 from Iran, at least 63 from Canada and 11 from Ukraine, according to officials.

Volodymyr Zelensky, Ukraine’s president, has said crash investigators from his country have arrived in Iran to assist the probe.

Mr Zelensky also said he plans to call Iran’s president Hassan Rouhani to discuss the crash and investigation.

“Undoubtedly, the priority for Ukraine is to identify the causes of the plane crash,” he said.

“We will surely find out the truth.”

Although Ukrainian officials initially agreed with suspicions that the three-year-old plane was brought down by mechanical problems, they later backed away from that view and declined to offer a cause while the investigation is ongoing.

The disaster occurred immediately after Iran launched a ballistic missile attack against Iraqi military bases housing US troops in retaliation for the killing of military general Qassem Soleimani last week.

Many of the victims are thought to be international students attending universities in Canada who were making their way back to Toronto via Kyiv after visiting family.

Justin Trudeau, Canada’s prime minister, has said 138 passengers on the flight were bound for his country and vowed to get to the bottom of what happened.

The crash has ranked among the worst losses of life for Canadians in an aviation disaster.