The leaders of Canada, the US and UK say they believe an Iranian missile shot down a Ukrainian plane near Tehran, killing 176 people.

Canadian Prime Minister Justin Trudeau has said he has intelligence that indicates the Ukraine International Airlines flight that crashed on Wednesday was shot down.

He described the crash as a "tragedy that not only shocked Canada, but the rest of the world".

Mr Trudeau added: "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. This may well have been unintentional.

Iran denies the reports, with government spokesman Ali Rabiei describing it as a "psychological warfare against Iran", according to state TV.

He added: "All those countries whose citizens were aboard the plane can send representatives and we urge Boeing to send its representative to join the process of investigating the black box".

Canadian Prime Minister Justin Trudeau responds to questions during a news conference in Ottawa, Canada | Image: Adrian Wyld/AP/Press Association Images

Mr Trudeau's comments came after US officials said they were "confident" the passenger plane that came down in Tehran was shot down by an Iranian missile, with President Donald Trump saying "somebody could have made a mistake".

One official said US satellites had detected the launch of two missiles shortly before the plane crashed, followed by evidence of an explosion, Reuters news agency reported.

Two other officials said Washington believed the downing of the Ukraine International Airlines plane was accidental.

All 176 people were killed in the crash - including 82 Iranians, 11 Ukrainians, 10 people from Sweden, four passengers from Afghanistan, three from Germany and four British nationals

Some 63 Canadians were among those who died, while 138 passengers on board the Boeing 737-800 plane were connecting to Canada.

Mr Trudeau also said Iran had indicated it will keep black boxes in Iran, but have told Ukraine's president that its investigators will be granted access.

He said his government will not rest until it gets closure, transparency, accountability and justice, but that it is too soon to be "apportioning blame for the crash" or drawing any conclusions.

"Canada is working with its allies to ensure that an in-depth investigation is conducted so that we can uncover the causes of this tragic crash," he said.

Flowers and candles are left among photographs at a memorial during a vigil in Toronto, Canada | Image: Chris Young/AP/Press Association Images

Iran's head of civil aviation has dismissed the idea the plane was hit by a missile as "impossible".

But European security sources have said they believe the US assessment is "credible".

In a news conference, US President Donald Trump said: "I have my suspicions - I don't want to say that because other people have those suspicions also. It's a tragic thing.

"But somebody could've made a mistake on the other side. It was flying in… not our system, it was nothing to do with us... it was flying in a pretty rough neighbourhood and somebody could've made a mistake.

"Some people think it was mechanical. I don't think that's even a question, personally."

US Secretary of State Mike Pompeo condemned Iran's missile attack in a call with Iraqi Prime Minister Adil Abdul-Mahdi on Thursday, according to the US State Department.

Mr Trump's comments came almost immediately after London said it is looking into "very concerning" reports the plane had been hit by a missile.

Fears were sparked by a Ukrainian official, who raised the prospect of the jet possibly having been targeted by an anti-aircraft missile.

Oleksiy Danilov, secretary of Ukraine's Security Council, told Ukrainian media officials had four working theories regarding the crash, including a missile strike.

Ukrainian investigators have arrived in Iran and are awaiting permission to examine the crash site and look for missile fragments.

"A strike by a missile, possibly a Tor missile system, is among the main (theories), as information has surfaced on the internet about elements of a missile being found near the site of the crash," Mr Danilov said.

A preliminary report by Iran's civil aviation authority, published on Thursday, said the crew did not make a radio call for help and were trying to turn back when it came down just outside Tehran Airport.

Investigators also said that the plane was engulfed in fire before it crashed.

It also confirmed that both of the black boxes that contain data and cockpit communications had been recovered, though they had been damaged and some data was lost.