A newly surfaced video appears to show the moment a Ukrainian airliner was hit by a missile before crashing not far from Iran's airport in Tehran on Wednesday.

The Ukrainian International Airlines flight PS752 bound for Kyiv crashed minutes after takeoff from the Imam Khomeini International Airport in Tehran on Wednesday amid escalating tensions between the United States and Iran.

All 176 passengers on board were killed.

The video, first reported by the New York Times newspaper and CNN, is consistent with statements made by Canadian, US and UK officials, who said intelligence indicated that an Iranian missile brought down the airliner, a conclusion dismissed by Iran.

The officials said it may have been a mistake.

The video shows a small explosion in the sky over Parand, the suburb where the Ukrainian airliner first stopped transmitting its signal, the New York Times reported.

The plane continues to fly for several minutes before turning back towards the airport, the newspaper said. It then exploded and crashed. A loud explosion can be heard.

Using geolocation technology and other techniques, Bellingcat, an independent international collective of researchers and citizen journalists, established the video was taken in Parand.

The New York Times and CNN said their news outlets were sent the video by internet-freedom researcher Nariman Gharib, who said he obtained it from another individual.

It is unclear why that person was recording at the time. Bellingcat reported it was possible that two missiles were fired, prompting the person filming to start recording. The New York Times also reported that the person started filming after hearing "some sort of shot fired".

Al Jazeera was unable to independently verify the video.

Iran has denied that a missile hit the airliner.

"All these reports are a psychological warfare against Iran ... all those countries whose citizens were aboard the plane can send representatives and we urge Boeing to send its representative to join the process of investigating the black box," Iranian state TV quoted government spokesman Ali Rabiei said in a statement.

Iranian Foreign Ministry spokesman Abbas Mousavi called on Canada to share the intelligence it had on the crash.

The crash happened hours after Iran launched missile attacks on US-led forces in Iraq, in retaliation for the US assassination of top Iranian commander, Qassem Soleimani.

Canadian Prime Minister Justin Trudeau on Thursday stressed the need for a "credible and complete investigation" and called for Iran to grant access to the crash site to Canadian and international investigators. There were 63 Canadians on board the plane.

'Planes don't just take off and explode'

Iran went on high alert early on Wednesday as Tehran sent missiles towards US troops in Iraq to the west and waited in vain for retaliatory fire from the US.

The governments of Iran, Ukraine and the US - where plane manufacturer Boeing Company is based - are now scrambling to determine exactly what brought down the plane as the tensions surrounding the tragedy upend international norms for investigating air disasters.

Some airline analysts agreed the evidence points to downing by an anti-aircraft missile.

Richard Aboulafia, aerospace analyst and vice president of the US-based Teal Group, told Al Jazeera that the possibility of the relatively new plane crashing "due to the aircraft or engine is between zero and one percent".

"Planes don't just take off and explode on their own," he added. "There is such a thing as uncontained turbine failure, but it does not look like this."

Aboulafia described the Ukrainian plane as "blowing up, like a grenade" and having "a lot of kinetics" that suggest a shootdown by anti-aircraft missiles.

"The mysterious Iranian government message saying this was mechanical failure was just bizarre," Aboulafia said, referring to the lack of pilot communication, black-box data or ground-control information before that announcement. "The optics speak of nothing good."

'Match the missile theory'

The optics may be shifting focus towards a downing, but the dearth of hard, verifiable information is frustrating some analysts.

"It might possibly be a terrorist act, but it's too hard to tell at this stage," said Geoffrey Thomas, an aviation expert at Airline Ratings.

Mark Zee, founder of Opsgroup, an aerospace intelligence firm, said that a clearer cause is emerging but is "still not concrete, because it's back-channel stuff to the media in the US".

"I suspect the US will have to come out with a statement on it, from the military side," he told Al Jazeera. "It sounds very likely that their radar and satellite picked events up that match the missile theory."

"Those systems are designed to look for anti-aircraft and missile activity," added Zee, referring to the timing of the incident among other factors such as the lack of a distress call to air-traffic control, an abrupt loss of navigation signals, no indications of an attempt to return to Tehran's airport and widely scattered wreckage.

"If the eyewitness video is verified, then that also lines up with the [missile] theory," Zee added. "Finally, the images from the Iranian news agency of the aircraft parts show something that looks a lot like projectile damage, but higher-resolution images would be needed."

Despite the US Federal Aviation Administration having successfully rerouted most air travel away from what had become a combat zone, Zee does not fault Ukraine International Airlines for flying in the area just hours after Iranian missiles were sent into Iraq. "You can't point any fingers there," said Zee.

Just after the crash, the Ukrainian embassy in Iran blamed technical failure - but then retracted the claim.

The incident is testing international protocols for multilateral cooperation when investigating airline disasters at a time of seriously heightened tensions between Tehran and Washington.

Iran formally invited the US National Transportation Safety Board (NSTB) to take part in its investigation into the crash, an Iranian official said on Thursday.

"The NTSB has replied to our chief investigator and has announced an accredited representative," Farhad Parvaresh, Iran's representative at the International Civil Aviation Organization, part of the United Nations, told Reuters news agency.

The NTSB declined to comment. A person briefed on the matter confirmed the NTSB had agreed to take part but said it was unclear what if anything its representative would be able to do under US sanctions.

Parvaresh said other countries including Ukraine and Canada had also been notified.

He denied US and Canadian claims that the jet had been shot down and said Iran was committed to a full and transparent investigation for the accident, which he described as a "tragedy and disaster" for everyone involved.

Under the rules of the 1944 Chicago Convention on International Civil Aviation that created a global regime for air travel overseen by the United Nations, Iran's air accident investigation branch automatically takes charge.

Ordinarily, because the jet was US-made, the US has the right to be accredited to the inquiry.

Black boxes from Boeing planes are often sent to Boeing's US suppliers, but that is looking unlikely in this case.

"We will not give the black boxes to the manufacturer (Boeing) and the Americans," Iran's Civil Aviation Organization chief Ali Abedzadeh told Iran's Mehr News Agency.

Iran could ask a third country to help analyse black-box data, especially if reading the information is technically difficult due to badly damaged casings.

Canada could end up playing a key role in the investigation.

On Thursday, Canada's Transportation Safety Board (TSB) announced that it has accepted Iran's invitation "to attend the accident site", and is making arrangements for its representatives to travel to Tehran. (Aljazeera)

• Rescue team carries a body at the site where the Ukraine International Airlines plane crashed

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