Doubts have been cast over Iranian assertions that a deadly Ukrainian plane crash near Tehran was the result of technical failures, with independent aviation operations experts saying a “shootdown” was the most likely explanation.

Iranian officials said technical issues were behind the Ukrainian International Airlines (UIA) crash, which happened shortly after take-off from Imam Khomeini International airport on Wednesday morning, killing all 176 people on board.

The Ukrainian embassy in Tehran initially echoed this stance, but later retracted its statement, and instead said it was for an official commission to determine the cause of the accident.

While some aviation experts said it was too early to speculate, the OPS group, an aviation risk-monitoring group, said: “We would recommend the starting assumption to be that this was a shootdown event, similar to MH17 – until there is clear evidence to the contrary,” highlighting photos of the crash site which they said “show obvious projectile holes in the fuselage and a wing section.”

UIA has also discounted the possibility of technical problems, insisting there was “nothing wrong” with the three-year-old Boeing 737-800, which had undergone a scheduled technical check only two days earlier.

“We guarantee the safety of our aircraft and the high qualification of our crews,” a spokesperson said.

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

Vadim Lukashevich, an independent Russian aviation expert, told The Independent that from evidence in the public domain, it was clear that there was some sort of fire on board, and that the plane broke up upon impact. The rest, he said, was “conjecture”.

“In any air accident, there are usually three factors: human, weather and mechanical. Here you can add a fourth – the political. We know that a few hours before the crash, Iran sent missiles over to US targets in Iraq. They were expecting a response by air. We know they were on full alert.”

It is too early to rule out an external strike, he said – even if Iranian claims that one of the engines caught fire turned out to be true.

“An engine fire does not exclude the possibility that it was caused by a missile strike,” he said. “The fact that Iran immediately discounted all explanations bar mechanical is suspicious.”

In light of the obvious political tensions in the area, it was a criminal error not to close off airspace to civil aviation, Mr Lukashevich added. “At a minimum, the Iranians tugged a tiger by his ear and didn’t run away.”

Iran has said it will not be handing over the plane’s black box recorder to US plane manufacturer Boeing.

Philip Baum, the editor of Aviation Security International magazine, said it was too early to speculate.

He told The Independent the incident was notable “given the fact it was in Iran and the timing, and if you associate two states with shootdowns, as victims, then Ukraine springs to mind purely because of MH17,” and Iran, given the 1988 US shooting down of an Iranian civilian plane.

But, he added, it could just be “a bizarre coincidence that it should happen at this moment of time”.

“I just think there is nothing to indicate that it was a shootdown, and if it was shot down then by whom?”

Zeev Sarig, the former head of Ben Gurion airport in Israel, said a technical failure could be behind the plane crash, or an explosive device on board.

Speaking to Russian news agency RIA, he said that while a full investigation was necessary, the two main possibilities were, “a bomb on board that runs on a timer or altitude monitor, exploding when the plane reaches a certain height” or “a technical malfunction about which we don’t know anything yet. Unfortunately from what I see that looks less likely.”

Qassem Biniaz, a spokesperson for Iran’s road and transportation ministry, said the pilot “lost control of the plane” after a fire broke out in one of its engines.

The aircraft is a predecessor to the Boeing 737 Max 8, which has been grounded for 10 months following the Lion Air crash in 2018 and the Ethiopian Airlines crash in 2019.

Faulty flight-control software on board those two Boeing 737 Max 8 airliners resulted in the planes nose-diving, leading to the deaths of 346 people.

Airline officials said most of the passengers were en route to the Ukrainian capital, Kiev, transiting through there to other destinations.

Three Britons were among those who died.