If the investigation into the crash of a Ukrainian airliner in Iran concludes that an Iranian missile was to blame it could inflame already dangerous tensions between Tehran and Washington.

The Iranian authorities have dismissed as "psychological warfare" claims by US and Canadian officials that Iran shot the Ukraine International Airlines plane out of the sky with a surface-to-air missile by accident.

They have said initial signs were that the airliner suffered a technical failure and was in the process of turning around, shortly after take-off from Tehran's international airport, when it crashed in flames early on Wednesday morning.

Trudeau: Evidence that plane was hit by missile

That is not what intelligence gathered by the US, Canada and Britain would suggest.

Canadian Prime Minister Justin Trudeau, whose country lost 63 of its own citizens among the 176 passengers and crew that died, was clear in his assessment.


"We have intelligence from multiple sources, including our allies and our own intelligence. The evidence indicates that the plane was shot down by an Iranian surface-to-air missile," he told a news conference in Ottawa on Thursday.

"This may well have been unintentional. This new information reinforces the need for a thorough investigation into this matter," he continued.

The timing of the crash - just hours after Iran hit US targets in Iraq with a barrage of ballistic missiles - had appeared to be an extraordinary coincidence.

Speculation swirled online that the Iranian explanation of the failure of the Boeing 737-800 was masking a far more uncomfortable truth - that some kind of missile caused the disaster.

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Iran so far is adamant that such claims are nonsense. Perhaps it will maintain that position even if evidence uncovered by international investigators demonstrates otherwise.

That is the position adopted by Russia after Russia-backed forces in eastern Ukraine shot down a Malaysian passenger jet over the territory six years ago.

Moscow still maintains its innocence even as the investigative team charge a number of Russians with involvement.

Officials say crash caused by Iranian missile

Or maybe Iran will accept the findings if they turn out to prove a missile took the plane out the sky, but blame the US for creating the conditions for the accident by triggering a sudden sharp escalation in tensions between the two foes with the assassination last week of Major General Qassem Soleimani, Iran's top military commander.

The Iranian military struck US targets in Iraq with missiles in revenge.

President Donald Trump stepped back from the brink of war with Tehran on Wednesday following the attack, choosing not to retaliate with US missiles even though he had previously warned that he would fire back.

Image: Plane engine among debris

Before the climbdown, the threat of a US retaliation would almost certainly have meant Iran's air defence systems would have been on high alert, ready to fire at hostile, incoming objects.

That would not explain why one of the missiles was potentially used against a civilian airliner, but it would have created the circumstance where such a misfire might have been more likely to happen than on an ordinary early morning.

There is another possible outcome of this tragedy if a missile is found to have been the cause.

The moment Ukrainian plane comes down in Iran

The enormity of such a senseless loss of life might prompt Iran and the US to work harder to find some kind of way forwards that calms the hostilities.

That is in part what happened back in 1988 when a US warship accidentally shot down an Iranian passenger jet, killing all 290 people on board.

At the time the US and Iran were engaged in the so-called tanker wars in the Gulf. The tragedy helped to precipitate an end to that conflict.