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.

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.

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.

'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.

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."

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.

"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. —AFP