The strike, which killed all 176 people aboard, could well have been a mistake amid intentional airstrikes and high tensions throughout the region, they say.

The crash came just hours after Iran launched a ballistic attack against Iraqi military bases housing US troops amid a confrontation with Washington over the US strike that killed an Iranian Revolutionary Guard general.

Four US officials, speaking on condition of anonymity, said on Thursday they had no certain knowledge of Iranian intent and the airliner could have been mistaken for a threat.

Canadian Prime Minister Justin Trudeau, whose country lost at least 63 citizens in the downing, said in Toronto: “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.”

Earlier on Thursday, President Donald Trump suggested he believed Iran was responsible, but wouldn’t directly blame the Iranians.

He dismissed Iran’s initial claim it was a mechanical issue.

“Somebody could have made a mistake on the other side,” Trump said, noting the plane was flying in a “pretty rough neighbourhood”.

“Some people say it was mechanical,” he added. “I personally don’t think that’s even a question.”

The US officials wouldn’t say what intelligence they had that pointed to an Iranian missile.

However they acknowledged the existence of satellites and other sensors in the region, as well as the likelihood of communication interceptions and other similar intelligence.

Two additional US officials said intelligence pointing to likely Iranian responsibility became clearer overnight.

It was not immediately clear how the US and its allies would react to the downing of the airliner. At least 63 Canadians and 11 Ukrainians were among the dead.

Despite efforts by Washington and Tehran to step back from the brink of war, the region remained on edge after the killing of Iranian general Qasem Soleimani and Iran’s retaliatory missile strikes.

US troops were on high-alert.

The latest assessment comes just a day after Defense Secretary Mark Esper and Army General Mark Milley said they hadn’t had a chance to review the intelligence on the incident.

A preliminary Iranian investigative report released on Thursday said the airliner pilots never made a radio call for help and the aircraft was trying to turn back when it went down.

Ukraine, meanwhile, said it considered a missile strike one of several theories.

The Iranian report suggests a sudden emergency struck the Boeing 737 operated by Ukrainian International Airlines just minutes after taking off from Imam Khomeini International Airport in Tehran.

Investigators from Iran’s Civil Aviation Organization offered no immediate explanation, however.

Iranian officials initially blamed a technical malfunction, something backed by Ukrainian officials before they said they wouldn’t speculate.

Before the US assessment, Iran’s IRNA news agency quoted Hasan Rezaeifa, the head of the of civil aviation accident investigation commission, claiming “the topics of rocket, missile or anti-aircraft system is ruled out”.

The Ukrainian airliner took off at 6.12 am on Wednesday, Tehran time, after nearly an hour’s delay.

It gained altitude heading west, reaching nearly 8000 feet, according to both the report and flight-tracking data.

Then something went wrong, though “no radio messages were received from the pilot regarding unusual situations,” the report said.

Eyewitnesses, including the crew of another flight passing above, described seeing the plane engulfed in flames before crashing at 6.18 am.

The crash caused a massive explosion when the plane hit the ground, likely because it had been fully loaded with fuel for the flight to Kyiv, Ukraine.