Iranian authorities have allegedly been caught clearing debris from the crash site of Ukrainian International Airlines Flight 752, where 176 people died, including 63 Canadians.

Canadian Prime Minister Justin Trudeau has confirmed he has evidence indicating the Boeing 737 was shot down by Iran, possibly by accident, in the hours after the Iranian missile attack on US targets in Iraq.

Iran has denied the accusation and is sticking to its story about the plane having mechanical issues. A preliminary report into the crash by the Iranian Civil Aviation Organisation said flight PS752 crashed after the pilot had tried to turn back to Tehran airport following a technical problem.

Open source investigation website Bellingcat, whose painstaking work linked downed MH17 to Russia back in 2014, said it had verified images of a bulldozer clearing a debris field in Shahedshahr on the outskirts of Tehran.

There are now concerns evidence has been compromised, contaminated, or worse, destroyed, and the truth about how and why the aircraft crashed may never be known.

“Those images (of the bulldozer) we were able to geolocate to the crash site,” Bellingcat investigator Giancarlo Fiorella told Channel 4 News.

“I found them to be really distressing because this is potentially the scene of a crime. If this was a shoot-down event, you don’t want to disturb the crash site before a thorough investigation can be conducted and I’m not sure one had been conducted.

“The presence of heavy machinery at the site and the bulldozing is very distressing.”

It follows news the two black boxes, which contain data and cockpit communications from the plane and are meant to be indestructible, had been damaged. Iranian aviation authorities claim parts of their memory had been lost.

Just before the crash, US intelligence reportedly picked up signals of radar systems on the ground being turned on. Satellites detected infra-red blips of two missile launches and a subsequent explosion.

The weapon that knocked the plane out of the sky is believed to be a Russian-built, surface to air Tor missile. Two images purporting to show the damaged head of the missile at the crash site have been circulating on social media.

“We are trying to verify a couple of images that would point to the involvement of a missile,” Mr Fiorelli said.

“There are in particular two images of what looks like a missile fragment. We’ve been working to determine their authenticity over the last couple of hours. They could be related to this event but they could also be from anywhere else in the world.

“It could be from another place in Iran, maybe they’re from that area but they’re not related to that event … Bellingcat works with open source information and we have lots of pictures video that show all kinds of damage to the aircraft, particularly around the engines.

“They are going to be really important in the investigation into this event.”

The British Government said it was looking into “very concerning reports” the jet was shot down.

Iran appeared to deny the claims, alluding to “doubtful scenarios” and calling on Canada to share intelligence Ottawa says suggests the airliner was mistakenly downed by an Iranian missile.

The Iranian foreign ministry invited the plane’s manufacturer Boeing to take part in the official inquiry into the crash.

Prime Minister Scott Morrison told radio station 2GB this morning that the plane tragedy was a “terrible accident”.

“I can confirm that we’ve had similar intelligence as our partners have,” he said.

“What I would say is that this is not a deliberate attack, as we can determine. It’s a terrible accident. What’s important is the co-operation with a full inquiry into what’s occurred there.

“This is a terrible tragedy.”

The Ukraine International Airlines jet was en route to the Ukrainian capital of Kiev with 167 passengers and nine crew members from several countries when disaster struck on Tuesday local time. There were no survivors.

After Iran, Canada copped the heaviest death toll, with 63 citizens on board. Many were believed to be Candaian Iranian students returning to university after the holidays.

American officials reportedly told Newsweek that the Pentagon’s assessment is that the incident was accidental with the airliner possibly mistaken for a threat.

Australia has an embassy in Tehran and is working with the Canadian Government

to support families of those killed in the crash.

“It is a terrible, terrible event and we’ll provide whatever support we can,” Mr

Morrison said.

The Prime Minister said it was critical a “full and transparent” investigation took place.

“That would include undertaking all efforts to ensure we get recovery of the black box recorder that can obviously inform that investigation,” he said

Australian Federal Police officers helped investigate the 2014 downing of Malaysian Airlines flight MH17 over eastern Ukraine.

Mr Morrison would not clarify exactly what assistance Australia was providing following this week’s crash, instead saying Canada had an “open invitation” to whatever help it required.

He said he had not spoken to Mr Trudeau and also plans to speak with his Ukrainian counterpart.

The crash came just a few hours after Iran launched a ballistic missile attack against Iraqi military bases housing American troops amid a confrontation with Washington over the US drone strike that killed Iranian Revolutionary Guard general Qassan Soleimani.

With AAP