The US has promised “appropriate action” in response to its assessment that an Iranian missile was responsible for downing a Ukrainian airliner that crashed outside Tehran.
The Iranian government has denied playing a role in the killing of all 176 people on board.
Secretary of state Mike Pompeo became the highest-level US official to directly pin the blame on Iran, after Canadian, Australian and British leaders announced similar intelligence conclusions on Thursday.
“We do believe it is likely that that plane was shot down by an Iranian missile,” he said.
Mr Pompeo said an investigation would continue into the incident and that once it was complete he was “confident that we and the world will take appropriate action as a response”.
Leaders said the plane appeared to have been unintentionally hit by a surface-to-air missile.
Earlier, Iran denied Western allegations that one of its own missiles downed the airliner that crashed early on Wednesday, hours after Iran launched more than a dozen ballistic missiles at two US bases in Iraq to avenge the killing of its top general in an American air strike last week.
“What is obvious for us, and what we can say with certainty, is that no missile hit the plane,” Ali Abedzadeh, head of Iran’s national aviation department, told a press conference.
“If they are really sure, they should come and show their findings to the world” in accordance with international standards, he added.
Hassan Rezaeifar, the head of the Iranian investigation team, said recovering data from the black box flight recorders could take more than a month and that the entire investigation could stretch into next year.
He also said Iran may request help from international experts if it is not able to extract the flight recordings.
The ballistic missile attack on the bases in Iraq caused no casualties, raising hopes that the standoff over the killing of General Qassem Soleimani would end relatively peacefully, though Iran has sent mixed signals over whether its retaliation is complete.
If the US or Canada were to present incontrovertible evidence that the plane was shot down by Iran, even if unintentionally, it could have a dramatic impact on public opinion in Iran.
The Iranian public had rallied around the leadership after the killing of Gen Soleimani last Friday, with hundreds of thousands joining the general’s funeral processions in several cities, in an unprecedented display of grief and unity.
But sentiments in Iran are still raw over the government’s crackdown on large-scale protests late last year sparked by an economic crisis exacerbated by US sanctions.
Several hundred protesters were reported to have been killed in the clampdown.
Those fissures could quickly break open again if Iranian authorities are seen to be responsible for the deaths of 176 people, mainly Iranians or dual Iranian-Canadian citizens.
Iran still points to the accidental downing of an Iranian passenger jet by US forces in 1988 – which killed all 290 people aboard – as proof of American hostility.
US, Canadian and British officials said on Thursday it is “highly likely” that Iran shot down the Boeing 737, which crashed near Tehran.
US officials said the airliner might have been mistakenly identified as a threat.
Canadian Prime Minister Justin Trudeau, whose country lost at least 63 citizens in the downing, said “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,” he said.
The US officials did not say what intelligence they had that pointed to an Iranian missile, believed to be fired by a Russian Tor system, known to Nato as the SA-15.
But they acknowledged the existence of satellites and other sensors in the region, as well as the likelihood of communication interceptions and other similar intelligence.
Western countries may hesitate to share information on such a strike because it comes from highly classified sources.
Videos verified by the Associated Press appear to show the final seconds of the ill-fated airliner, which had just taken off from Iran.
In one video, a fast-moving light can be seen through the trees as someone films from the ground.
The light appears to be the burning plane, which plummets to the earth as a huge fireball illuminates the landscape.
Someone off-camera says in Farsi: “The plane has caught fire. … In the name of God, the compassionate, the merciful. God, please help us. Call the fire department!”
Ukraine’s President Volodymyr Zelenskiy said “the missile theory is not ruled out, but it has not been confirmed yet”.
In a Facebook post, he reiterated his call “on all international partners” – the US, Britain and Canada in particular – to share data and evidence relevant to the crash.
He also announced plans to discuss the investigation with Mr Pompeo.
Ukraine’s foreign minister Vadym Prystaiko tweeted that he and the president met US embassy officials on Friday and obtained “important data” about the crash.
The minister did not specify what kind of data it was, but said it would be “processed by our specialists”.
In an interview with Fox News’s Laura Ingraham that aired late on Thursday, Mr Pompeo said commercial airliners need to know if it is safe to fly into and out of Tehran.
“If the international community needs to shut down that airport, so be it,” he said. “We need to get to the bottom of this very, very quickly.”
Germany’s Lufthansa airline said it and subsidiaries are cancelling flights to and from Tehran for the next 10 days as a precautionary measure, citing the “unclear security situation for the airspace around Tehran airport”.
Other airlines have been making changes to avoid Iranian airspace.
Britain’s Foreign Office has advised against all travel to Iran, and against all air travel to, from or within the country.