Mourners light candles for the victims of Ukrainian Airlines flight 752 which crashed in Iran during a vigil at Mel Lastman Square in Toronto, Canada on Friday. Photo: AFP

Iran's civil aviation chief denied Friday that a missile downed a Ukrainian airliner which crashed killing all 176 on board, dismissing claims of a catastrophic mistake by Tehran's air defenses.

The declaration at a press conference in the Iranian capital came after Britain and Canada pointed to what they said were signs of an accidental missile strike.

"One thing is for certain, this airplane was not hit by a missile," Iran's civil aviation chief Ali Abedzadeh said.

The Boeing 737 crashed on Wednesday shortly after Tehran launched missiles at US forces in Iraq in response to the killing of a top Iranian general in a US drone strike in Baghdad.

It is Iran's worst civil aviation disaster since 1988 when the US military said it shot down an Iran Air plane by mistake, killing all 290 people on board.

Canadian Prime Minister Justin Trudeau said Thursday that multiple intelligence sources indicated that an Iranian missile downed Ukraine International Airlines Flight PS752 after it took off from Tehran.

But Abedzadeh rejected the allegation, saying that "any remarks made before the data is extracted (from the plane's black box flight recorders) ... is not an expert opinion."

Trudeau's comments 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 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 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 said late Thursday it had received formal notification of the crash from Iran and would send a representative to join the investigation.

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

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.

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