Canadian Prime Minister Justin Trudeau says evidence shows the Ukrainian airliner that crashed in Tehran on Wednesday, killing all 176 people on board, was shot down by an Iranian surface-to-air missile.
Citing "multiple sources, including our allies and our own intelligence," Mr Trudeau said the evidence "indicates that the plane was shot down by an Iranian surface-to-air missile".
Mr Trudeau said the shooting down of the jet "may well have been unintentional".
Earlier on Thursday US officials, quoted by several outlets, said they believe the Ukrainian International Airlines Boeing 737-800 was hit by a missile.
They said they had identified the signature of an anti-aircraft missile battery that was activated shortly before the aircraft crashed. They also said they had identified the infrared signature of two suspected missile launches.
“Somebody could have made a mistake on the other side," US President Donald Trump said, adding that the plane was flying in a “pretty rough neighbourhood."
“Some people say it was mechanical,” he added. “I personally don't think that's even a question."
Tehran doubled down on denying the plane had been shot down.
Iranian government spokesman Ali Rabiei dismissed reports that the airliner had been brought down by a surface-to-air missile as "psychological warfare against Iran".
Mr Rabiei urged Boeing to send a representative to "join the process" of investigating the aircraft's damaged flight recorders, which were recovered on Wednesday.
Iran had said it would not send the boxes to the US for inspection despite US calls for co-operation.
Oleksiy Danylov, secretary of Ukraine's national security council, said prior to the reports on Thursday the country was looking at various possible causes for the crash including a possible missile attack, a collision, an engine explosion or terrorism.
The incident happened early on Wednesday morning, moments after the flight took off from Tehran.
Ukrainian President Volodymyr Zelenskiy said the team including search and rescue experts would be involved in the identification and repatriation of those killed. Some of the experts were also involved in the investigation of Malaysia Airlines Flight MH17 that was brought down over Ukraine in 2014.
Canadian as well as Ukrainian leaders have said answers must be given to questions that hang over Ukrainian International Airlines Flight 752.
Mr Trudeau said his nation expects to have a role in the investigation.
“Canadians will be asking for answers and we’re there to support and find out the truth,” Mr Trudeau said as he expressed his condolences.
Of the 176 people on board, at least 63 of the passengers were Canadian citizens. Most of the passengers, 138 people, were supposed to board connecting flights to Canada, which is home to a large Iranian diaspora.
According to the initial Iranian report into the crash, the crew of a Ukrainian airliner was trying to return back to Imam Khomeini International Airport in Tehran when it went down.
Witnesses, including the crew of a passing flight, described seeing the plane engulfed in flames before crashing, the report said. The crash caused a massive explosion, likely to have been because the aircraft was fully loaded with fuel for its flight.
Questions have been raised over the circumstances of the crash. The plane went down just hours after Iranian ballistic missiles struck US and coalition bases in Iraq.
Aviation experts have been left grappling for answers in the wake of the crash. "To me, it has all the earmarks of an intentional act. I don't know whether it was a bomb or a missile or an incendiary device," a former Federal Aviation Administration accident investigation chief, Jeff Guzzetti, told The Washington Post.
"I just know aeroplanes don't come apart like that," Mr Guzzetti said.