A Ukraine airliner that crashed in Iran, killing all 176 people on board, was most likely brought down accidentally by Iranian air defences, US officials reportedly said on Thursday, a conclusion Iran dismissed as "impossible".

The Ukrainian International airliner 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 United States and Iran.

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One US official told Reuters News Agency that US satellites had detected the launch of two missiles shortly before the plane crashed, followed by evidence of an explosion. Two officials said Washington believed the downing of the plane was accidental. Newsweek, CBS and the Associated Press news agency also reported the US officials' assessments.

The Pentagon declined to comment.

Iran reportedly 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.

Ukrainian President Volodymyr Zelenskyy lays flowers to commemorate victims of the Ukraine International Airlines Boeing 737-800 plane crash, at a memorial in Boryspil International airport [Handout/Ukrainian Presidential Press Service/Reuters]

Iranian investigators earlier on Thursday said the airliner was on fire and tried turning back but its crew never made a radio call for help.

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

The plane carried mostly Iranians and Iranian Canadians.

Later on Thursday, Canadian Prime Minister Justin Trudeau said Ottawa also had intelligence from multiple sources indicating that the Ukrainian airliner was mistakenly shot down by Iran.

"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.



"This may well have been unintentional," Trudeau added in a press conference.

Ukraine examining four scenarios

Ukraine, meanwhile, outlined four potential scenarios on Thursday to explain the deadly crash, including a missile strike and terrorism.

Oleksiy Danilov, Ukraine's top security official, said that airliner may have been brought down by a Russian-made missile.

Rescue team carries a body at the site where the Ukraine International Airlines plane crashed after takeoff from Iran's Imam Khomeini airport [Nazanin Tabatabaee/WANA/Reuters]

According to Danilov, who is the secretary of Ukraine's National Security and Defence Council, a team of 45 Ukrainian experts had arrived in Tehran to examine why the plane crashed.

Back in Washington, US President Donald Trump said he had "suspicions" about the crash.

"It was flying in a pretty rough neighbourhood and somebody could have made a mistake," he said.

"Some people say it was mechanical. I personally don't think that's even a question," he added, without offering any evidence.