Updated 10.15am with Iran's reaction

Multiple intelligence sources indicate that Iran shot down a Ukrainian airliner after it took off from Tehran, killing all 176 on board, including 63 Canadians, according to Prime Minister Justin Trudeau.

Reacting, Iran's civil aviation chief Ali Abedzadeh has insisted that the plane was not hit by a missile.

Trudeau's comments came as video emerged that appeared to show the moment the airliner was hit. 

That and other footage posted on social media increasingly pointed to a catastrophic mistake by Tehran's air defense batteries in bringing down Ukraine International Airlines Flight PS752 early Wednesday.

The video, which The New York Times said it verified, shows a fast-moving object rising at an angle into the sky before a bright flash is seen, which dims and then continues moving forward. Several seconds later an explosion is heard.

Citing information from allies as well as Canada's own intelligence, Trudeau said the plane appeared to have been hit by an Iranian surface-to-air (SAM) missile.

"We know this may have been unintentional. Canadians have questions, and they deserve answers," Trudeau told reporters.

He was backed by other Western leaders, including British Prime Minister Boris Johnson who said mounting evidence supported a missile strike, which "may well have been unintentional."

US President Donald Trump indicated that Washington officials believed the Kiev-bound Boeing 737 was struck by one or more Iranian missiles before it ditched and exploded outside Tehran.

The US National Transportation Safety Board late Thursday said it had received formal notification of the crash from Iran and would send a representative to join the crash probe.

Iran's foreign ministry earlier invited the US plane maker Boeing to "participate" in the inquiry.

The flight went down in the dark just minutes after takeoff, with no radio message from the pilot to indicate distress, according to the Iranian Civil Aviation Organization.

It was carrying 82 Iranians, 63 Canadians, 11 Ukrainians, 10 Swedes, four Afghans, three Germans and three Britons.

With tensions high between the United States and Iran, the disaster unfolded just hours after Tehran launched ballistic missiles towards bases in Iraq housing US troops.

Iran retaliated for the January 3 US drone strike in Baghdad that killed a top Iranian general.

The Iranian government said the missile strike scenario made "no sense," however, arguing that several internal and international flights had been sharing approximately the same airspace.

Tehran later asked Ottawa to share its information with Iranian investigators.

Iran civil aviation boss 'certain' Ukraine plane not hit by missile

Iran's civil aviation chief Ali Abedzadeh said Friday he was "certain" a Ukrainian airliner which crashed outside Tehran this week was not hit by a missile.

"One thing is for certain, this airplane was not hit by a missile," Abedzadeh told a news conference in Tehran after Britain and Canada both said intelligence sources suggested a catastrophic error by Iranian air defence batteries had downed the aircraft.

"The information in the black boxes ... is crucial for the aviation organisation to make a statement," Abedzadeh said, adding that they were intact and under examination.

Dismissing allegations against Iran, he said that "any remarks made before the data is extracted ... is not an expert opinion."

The statements from Britain and Canada came as video footage emerged that appeared to show the moment the airliner was hit.

The footage, which The New York Times said it had verified, shows a fast-moving object rising to an angle into the sky before a flash is seen, which dims and then continues moving forward. Several seconds later an explosion is heard.

"We have seen some videos. We confirm that the airplane was on fire for 60 to 70 seconds," Abedzadeh said.

But "that it was hit by something cannot be scientifically correct," he added.

'Canadians want answers'

Trudeau said Canada was working with allies to ensure a credible probe.

"The families of the victims want answers, Canadians want answers, I want answers," he said.

"This government will not rest until we get that."

Canada's transportation safety board on Thursday said it had accepted an invitation from Iran's civil aviation authority to join the inquiry.

Britain's Johnson called Thursday for a full, transparent investigation.

'I have my suspicions'

Trump would not directly confirm what US intelligence was saying privately.

"I have my suspicions," Trump said, adding that "somebody could have made a mistake."

But unnamed officials told US media that satellite, radar and electronic data indicated Tehran's air defense units downed the aircraft.

ABC News reported that an unnamed official said it was "highly likely" the plane was brought down by two SAMs.

Black boxes

Ukraine called for United Nations support for a broad investigation, and sent 45 crash investigators to Tehran to take part in the inquiry led by Iranian authorities. 

Investigators are pursuing several possibilities, including engine failure, a missile strike or an act of terror.

"If any country has information that can help conduct a transparent and objective investigation into the tragedy, we are ready to receive it and cooperate in further verification," the Ukraine presidency said in an English-language statement.

Ali Abedzadeh, head of Iran's civil aviation organization and deputy transport minister, said Iran and Ukraine were "downloading information" from the aircraft's black boxes retrieved from the crash site. 

"But if more specialized work is required to extract and analyze the data, we can do it in France or another country," he said.

Analysts were examining photographs posted online of the wreckage and a private video apparently taken of the flight when it was struck for evidence that it was downed by a missile.

"I think this has a very good possibility of being accurate," John Goglia, a former US aviation safety expert on the National Transportation Safety Board, said of the missile theory. 

"Airplanes that have just taken off and have made a climb to 8,000 feet, that's entering the safest period of time in the flight. So even an engine failure at that altitude should not cause the type of event we've just observed," he told AFP.

The Ukrainian airline crash brought back memories of another tragedy, involving a US military error. 

In 1988, an Iran Air flight was mistakenly shot down over the Gulf by a surface-to-air missile fired from the US warship USS Vincennes. 

All 290 people aboard, most of them Iranians, were killed.