An engine problem may have been to blame for a Ukrainian plane crash near Tehran which killed all 176 people on board, intelligence officials say.

The Ukraine International Airlines Boeing 737-800 crashed just hours after Iran fired missiles at bases housing US forces in Iraq, prompting speculations that the aircraft may have been shot down.

However five security officials, including one European, one Canadian and three Americans, told Reuters an initial assessment by western intelligence agencies had shown the jet suffered a technical malfunction.

The Canadian source, who did not want to be named, said there was evidence one of the jet’s engines had overheated.

However, the maker of the plane’s engines, French-US firm CFM, co-owned by General Electric Co and France’s Safran, said it was too soon to speculate about the cause of the crash.

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

Earlier on Wednesday, independent aviation operations experts said a “shootdown event” may have been the true explanation.

The Ukrainian embassy in Tehran initially attributed the crash to engine failure but later retracted the statement and instead said it was for an official commission to determine the cause.

Flight PS752 took off from Tehran’s Imam Khomeini International Airport at 06.12 local time [03.12 GMT], losing contact just two minutes later.

It came just hours after Iran fired ballistic missiles at two bases used by American coalition forces in Iraq in retaliation for the US airstrike that killed Iranian military commander Qassem Soleimani on Friday.

Airline officials said most of the passengers were on their way to the Ukrainian capital, Kiev, where they were planning to go on to other destinations.

There were 167 passengers and nine crew members on board, according to aviation authorities.

Among the victims were three Britons, 82 Iranians, 63 Canadians, and 11 Ukrainians.

The British victims were named as engineers Sam Zokaei, from Surrey, Saeed Tahmasebi Khademasadi, from west London, and Mohammad Reza Kadkhoda Zadeh, from Brighton.

A newlywed Canadian couple, who had travelled to Iran to get married, were also on the plane with four members of their wedding party when it went down.

Iran has refused to hand over the plane’s black boxes – which contain key flight data and cockpit voice recordings – to US-based Boeing or US authorities.

Under International Civil Aviation Organisation rules, investigations into air crashes are led by the country where the accident took place, in this case Iran, but other countries may be involved, including the country of the manufacturer.

Ali Abedzadeh, head of the Iran Civil Aviation Organisation, said it was unclear which country the black box would be sent to for investigation, although Ukraine is expected to be involved in the process.

The crash came after Boeing grounded its 737 MAX fleet after two crashes in 2018 and 2019. The 737-800 is one of the world’s most-flown models with a good safety record and does not have the software feature implicated in crashes of the 737 MAX.