Canadian Prime Minister Justin Trudeau said on Thursday that evidence indicates an Iranian missile downed a Ukrainian airliner on Wednesday and that the strike "may have been unintentional".

Trudeau said Canadian and allied intelligence supports that cause. He declined to get into the specific intelligence, but said it appeared it was a surface-to-air missile that struck the plane.

"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," Trudeau told reporters in Ottawa. "This may well have been unintentional."

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Earlier, United States officials said it was “highly likely” that an Iranian anti-aircraft missile downed the airliner, killing all 176 people on board. They suggested it could well have been a mistake. British Prime Minister Boris Johnson also said there was a "body of information that the flight was shot down by an Iranian surface-to-air missile". He added "this may well have been unintentional".

Iran had earlier dismissed the reports as "illogical rumours".

"It is impossible that a missile hit the Ukrainian plane," Iran's head of civil aviation was quoted by Iran's INSA news agency as saying.

The Ukraine International Airlines plane bound for the Ukrainian capital, Kyiv, crashed minutes after takeoff from the Imam Khomeini International Airport in Tehran on Wednesday amid escalating tensions between the US and Iran. There were 63 Canadians on board.

Iranian investigators released an initial report on Thursday saying that the airliner was on fire and tried turning back but that its crew never made a radio call for help. That report characterised the incident as an accident. In-depth investigations into airline disasters generally take months.

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

Renewed calls for Canadian investigation

Trudeau 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. So far, Iran has only announced an agreement with Ukraine, whose investigators arrived in the country on Thursday.

In a Wednesday night phone call, Canadian Foreign Minister Francois-Philippe Champagne pushed his Iranian counterpart Mohammad Javad Zarif to allow Canada access "to provide consular services, help with identification of the deceased and take part in the investigation of the crash".

Trudeau said that the conversation is ongoing, but attaining such access might prove difficult as Canada closed its embassy in Iran in 2012, expelled Iranian diplomats from Canada and suspended all diplomatic relations.

In the meantime, Italy and other allies with a diplomatic presence in the Islamic Republic are helping Canada with consular assistance for friends and families of the Canadian victims. Canada is also remaining in close contact with Ukraine as they conduct the investigation, Trudeau said.

The prime minister added that Iran has so far declined to send two black boxes recovered from the wreckage to Canada for analysis. Instead, Iran has said that Ukrainian investigators will have access to the devices.

On Thursday, Canada's Transportation Safety Board (TSB) announced that it has accepted Iran's invitation "to attend the accident site".

"We have accepted this invitation and we are making arrangements to travel to the site. The TSB will be working with other groups and organizations already on site."