Downing Street is urgently looking into “very concerning” reports that the Iran plane crash was caused by an anti-aircraft missile.
The Prime Minister’s spokesman said it was vital that a “full and transparent” investigation was carried out into what brought down Ukrainian International Airlines flight 752 with the loss of 176 lives, including three Britons.
It came after Newsweek magazine in the US cited sources in the Pentagon, US Intelligence and Iraqi intelligence as saying the Boeing 737 was hit by an anti-aircraft missile.
It said a Russian-built Tor-M1 surface to air missile, fired accidentally by Iran, was the most likely cause of the disaster.
Ukrainian investigators said they would search the wreckage for missile fragments as they refused to rule out a terror attack as a cause of the crash.
The flight from Tehran to Kyiv crashed minutes after take off on Wednesday morning, killing all 176 people on board.
The Prime Minister’s spokesman said: “Reports we have seen are very concerning. We are urgently looking into them and we want to see a full, transparent investigation into what happened.
“The Prime Minister has offered support to Ukraine and will be with the Ukrainians on how best to help them.”
Mr Johnson spoke to the Ukrainian President Volodymyr Zelenskiy on Thursday and offered Britain’s condolences for the loss of 11 Ukrainians on board the aircraft.
Iranian aviation authorities on Thursday said initial enquiries suggested the aircraft caught fire and attempted to turn back to the airport before it nosedived into the ground, but that no radio messages were received from the crew.
“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” said Ali Abedzadeh , the head of Iran’s Civil Aviation Organisation (CAOI).
He said both black box flight recorders had been recovered but that they are partially damaged and some of their memory may have been lost.
The crash came just hours after Iran fired a barrage of ballistic missiles at US forces in neighbouring Iraq, fuelling speculation that the aircraft may somehow have been caught in the crossfire.
Oleksei Danilov, the secretary of Ukraine’s national security council, said investigators were considering several scenarios for Wednesday’s disaster including missile strike, a terrorist act committed on board the aircraft, engine failure, or collision with a drone or a bird, but that there was currently “no preferred theory.”
He said a 45-person strong Ukrainian team, including investigators who worked on Russia’s shoot down of Malaysian airlines flight MH17 in 2014, reached the crash site just outside Tehran on Thursday morning and would search for missile fragments.
“A strike by a missile, possibly a Tor missile system, is among the main (theories), as information has surfaced on the internet about elements of a missile being found near the site of the crash,” Danilov said.
He appeared to be referring to unverified photographs posted by a number of Iranian Twitter users of what appeared to be a missile nose cone.
The Tor M is a Russian-made anti-aircraft system used by Iran. Iran bought 29 of the systems from Russia as part of a $700 million contract signed in December 2005.
He said the team was in “effective diplomatic talks” with Iran on the joint investigation and had “every reason to hope for full cooperation on all questions including our team’s access to the black boxes.”.
Iran did not immediately respond to Mr Danilov’s comments. On Wednesday it dismissed speculation that its air defences may have shot down the aircraft by mistake.
Engine failure would be a blow for Boeing, which is already reeling from a series of crashes involving its 737-MAX aircraft. The aircraft that crashed in Iran was a far more reliable 737-800 model.
Ukraine’s president, Volodymyr Zelenskiy, declared a national day of mourning on Thursday.
The investigation is likely to be further complicated by the state of not-quite declared war between Iran and the United States.
Under the treaties that regulate international air travel, an air crash is investigated by the country where it occurred, in this case Iran.
In normal circumstances the United States Nation Transportation Safety Board, would also be involved in any investigation into the crash of a Boeing aircraft, but Iran has ruled out handing over black boxes to the US.
Canada, which lost at least 63 citizens on the flight, would also normally be expected to take part in the investigation but has had no diplomatic relationship with Iran since 2012.
However, the Iranian and Canadian foreign ministers held a rare telephone conversation on Wednesday night, signalling a potential thaw.
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-mast, and Mr Trudeau vowed to get to the bottom of the disaster.
“Know that all Canadians are grieving with you,” he said, addressing the victims’ families.
Professor Graham Braithwaite, an expert on aviation safety at Cranfield University, said investigators would be focused on extracting black box data to see if conversations in the cockpit revealed anything about the problem that hit the aircraft.
“Any investigation at this stage would be open minded to any and all of those theories. The fact someone has ruled that out one in the first few hours is not credible,” he said.