Iran has again rejected suggestions that one of its missiles brought down a Ukrainian passenger jet near the capital, Tehran, on Wednesday.
Its civil aviation chief said on Friday he was "certain" that the plane was not hit by a missile.
He was responding to claims by Western leaders that evidence suggested the plane had been hit by a surface-to-air missile, possibly in error.
New video appeared to show a plane being hit by a projectile over Tehran.
The crash of Ukraine International Airlines flight PS752 with the loss of 176 lives came just hours after Iran carried out missile strikes on two airbases housing US forces in Iraq.
US media have speculated that the airliner may have been mistaken for a warplane as Iran prepared for possible US retaliation.
Victims of the crash included 82 Iranians, 63 Canadians and 11 Ukrainians as well as nationals from Sweden, the UK, Afghanistan and Germany.
Iran has promised a full investigation. However, TV images from the crash site on Thursday showed a mechanical digger helping to clear debris away, raising concerns that important evidence could have been removed.
Meanwhile, the so-called "black box" recovered from the wreckage will be opened on Friday, Iran's official Irna news agency reported.
Iran said it would download the information itself, adding that the process could take up to two months.
Black boxes contain the flight data recorder and the cockpit voice recorder, and could provide vital clues about what caused the crash.
What does Iran say?
At a news conference on Friday, Iran's Civil Aviation Organisation (CAOI) chief Ali Abedzadeh repeated his view that a missile was not the cause of the crash.
"The thing that is clear to us and that we can say with certainty is that this plane was not hit by a missile," he told reporters.
"As I said last night, this plane for more than one and a half minutes was on fire and was in the air, and the location shows that the pilot was attempting to return."
On Thursday, government spokesman Ali Rabiei accused the US and its allies of "lying and engaging in psychological warfare" in their speculation over the cause of the accident.
An Iranian official told the BBC on Friday that there was documentation to prove that the plane had a mechanical issue before take-off. It was not signed off for flying, but Ukrainian airline officials had overruled these objections, the official said, without giving further details.
What has been said about a possible missile strike?
Canadian Prime Minister Justin Trudeau said he had received intelligence from multiple sources indicating the plane was shot down by an Iranian surface-to-air missile, adding that it was possible that this was unintentional.
"This reinforces the need for a thorough investigation," he said. "Canadians have questions and they deserve answers."
But he said it was too early to apportion blame or draw any conclusions, and refused to go into detail about the evidence.
The Ukrainian flight was headed to the Canadian city of Toronto via the Ukrainian capital, Kyiv.
UK Prime Minister Boris Johnson echoed Mr Trudeau's words and said Britain was working closely with Canada and other international partners affected by the crash.
Speaking in Canada, UK Foreign Secretary Dominic Raab said British nationals were advised not to travel to Iran, "given the body of information that UIA Flight 752 was shot down by an Iranian surface-to-air missile, and the heightened tensions".
Newsweek quoted a Pentagon and senior US intelligence officials, as well as an Iraqi intelligence official, as saying they believed flight PS752 was hit by a Russian-made Tor missile.
Ukraine said on Friday that the US had passed on "important data" about the crash to President's Volodymyr Zelensky. The president is due to speak later on Friday with US Secretary of State Mike Pompeo, a government spokesperson said.