Iran denied on Thursday that a Ukraine International Airlines plane that crashed near Tehran had been hit by a missile, while Canada and the United States said they have evidence indicating that Flight PS752 was shot down by an Iranian missile.

All 176 people on board the flight, including 63 Canadians, died in the crash of the Boeing 737-800.

"All these reports are a psychological warfare against Iran ... 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," Iranian government spokesperson Ali Rabiei said in a statement, according to state TV.

Canadian Prime Minister Justin Trudeau told reporters Thursday that Canada has intelligence from multiple sources that "indicates that the plane was shot down by an Iranian surface-to-air missile."

"This may well have been unintentional," Trudeau said.

"This new information reinforces the need for a thorough investigation into this matter. Canada is working with its allies to ensure that a thorough and credible investigation is conducted to determine the causes of this fatal crash. As I said yesterday, Canadians have questions and they deserve answers."

Iran, in response to Trudeau's statement, called on Canada to share its information with Tehran, foreign affairs spokesperson Abbas Mousavi was quoted as saying by state news agency IRNA.

Investigations on the cause of the Ukrainian plane crash have launched based on Int. standards & ICAO regulations; Ukraine and Boeing have been invited- as the owner & the manufacturer- to take part in it. We appreciate any country who can provide info to the Committee in charge. <a href="https://t.co/62UQsdSf6R">pic.twitter.com/62UQsdSf6R</a> —@SAMOUSAVI9

In light of the new information pointing to a possible shooting down of the plane, Britain advised its citizens against all travel to Iran.

"We also recommend against taking a flight to, from and within Iran," U.K. Foreign Secretary Dominic Raab added in a statement.

The Canadian government is advising Canadians to avoid all non-essential travel to Iran, and avoiding all travel to within 10 kilometres of the Iraq border and to the Sistan-Baluchistan province.

The Transportation Safety Board of Canada said it will go to the crash site after being invited by Iran's Aircraft Accident Investigation Bureau.

Iran also invited the U.S. National Transportation Safety Board (NTSB) to take part in its investigation into the crash, and the agency has agreed to assign an investigator, an Iranian official said.

A person briefed on the matter confirmed the NTSB had agreed to take part but said it was unclear what, if anything, its representative would be able to do under U.S. sanctions.

The U.S. is allowed to take part under global rules since the Boeing jet was designed and built there.

Iran's official news agency later reported that officials were inviting Boeing experts to join the investigation.

A team from Ukraine held discussions in Tehran on Thursday, Farhad Parvaresh, Iran's representative at the International Civil Aviation Organization, part of the United Nations, told Reuters.

Iran is ready to provide consular facilities and visas for accredited investigators, he added.

Video appears to show missile impact over Tehran

The plane, flying to Kyiv with mostly Iranians and Iranian-Canadians, crashed shortly after takeoff from Tehran's Imam Khomeini airport. Trudeau said Wednesday that 138 of the passengers were bound for Canada.

Tracking data suggests the flight climbed to just over 2,400 metres and reached a speed of 500 km/h before it plunged back to the ground, crashing near a soccer field and irrigation canal.

Video obtained by the New York Times appears to show a missile making impact on an aircraft over Tehran. The Times said visual and sonic clues in the footage match flight path information and satellite imagery of the area near where Flight PS752 crashed.

Watch footage of the purported missile strike:

Video recording appears to show a missile making impact on an aircraft over Tehran. The New York Times says visual and sonic clues in the footage match flight path information and satellite imagery of the area near where Flight PS752 crashed. (Video courtesy of the New York Times) 0:20

Both Reuters and The Associated Press reported that U.S. officials said satellite data led them to believe Flight PS752 was most likely shot down accidentally by Iranian air defence.

Two U.S. officials, speaking to The Associated Press on the condition of anonymity to discuss sensitive intelligence, said they had no certain knowledge of Iranian intent.

One U.S. official told Reuters that Flight PS752 had been tracked by Iranian radar and said satellite data showed the plane was airborne for two minutes after departing Tehran when the heat signatures of two surface-to-air missiles were detected.

That was quickly followed by an explosion in the vicinity of the plane, this official said. Heat signature data then showed the plane on fire as it went down.

Government sources told CBC News that Canadian officials were briefed on the U.S. intelligence at a meeting Thursday morning of the cabinet incident response group, with senior officials of the Privy Council Office, Department of National Defence, Transport Canada, Global Affairs and Public Safety.

Trump suspects crash was a mistake

The Pentagon declined to comment, but U.S. President Donald Trump on Thursday said the deadly crash could have been a mistake.

When asked about the crash at a White House news conference, he told reporters he had a terrible feeling about the downed airliner but provided no additional details.

Watch Trump discuss his 'suspicions' about the plane crash.

Donald Trump says someone on "the other side" could have made a mistake leading to the deadly plane crash in Iran 1:01

"Somebody could have made a mistake," he told reporters. "I have my suspicions."

The crash came just a few hours after Iran launched a ballistic missile attack against Iraqi military bases housing U.S. troops amid a confrontation with Washington over the U.S. drone strike that killed Iranian Revolutionary Guard Gen. Qassem Soleimani. Two U.S. officials said the airliner could have been mistaken for a threat.

The Iranian report said eyewitnesses, including the crew of another flight passing above, described seeing the plane engulfed in flames before crashing at 6:18 a.m. local time. Flight-tracking data for the plane stopped before the crash, which occurred in the town of Shahedshahr to the northeast of the plane's last reported position. That's the wrong direction of the flight plan, bolstering the idea that the pilots tried to turn the aircraft back to the airport.

'Elements of a missile' possibly spotted near crash site

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

This image from Maxar's WorldView-2 satellite shows the area where Flight PS 752 crashed after take-off from Tehran's Imam Khomeini airport. (Maxar Technologies/Handout via Reuters)

"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," Danilov said. He did not elaborate on where he saw the information on the internet.

Ukrainian investigators who arrived in Iran earlier on Thursday are awaiting permission from Iranian authorities to examine the crash site and look for missile fragments, Danilov said.

The Tor is a Russian-made missile system. Russia delivered 29 Tor-M1s to Iran in 2007 as part of a contract worth $700 million US that was signed in December 2005. Iran has displayed the missiles in military parades as well.