Ukraine and Canada agreed to push for an objective investigation into the crash of Ukraine International Airlines Flight PS752, President Volodymyr Zelensky said Friday after speaking with Prime Minister Justin Trudeau.

"There should not be speculation about the tragedy; Ukraine and Canada will use all possible means to advocate for an objective and comprehensive investigation," Zelensky wrote on Twitter.

Canada and others have said the plane was brought down by an Iranian missile, probably by mistake. All 176 people on board the flight died in the crash of the Boeing 737-800.

Ukraine's Ministry of Foreign Affairs initially said that 63 Canadians were killed, but Canadian Foreign Affairs Minister François-Philippe Champagne on Friday told reporters the number is now 57. Champagne said it is a "very fluid situation."

Iran will announce on Saturday the reason for the crash, according to the semi-official Fars news agency, citing an informed source.

The announcement will reportedly take place after a meeting of a commission focused on air accidents, but Fars did not provide any detail on what kind of information had been collected.

President <a href="https://twitter.com/ZelenskyyUa?ref_src=twsrc%5Etfw">@ZelenskyyUa</a> and I spoke on the phone again today. We talked about the need to work closely together throughout the investigation so that the families of the victims can get the answers they deserve. More: <a href="https://t.co/29B90KdP92">https://t.co/29B90KdP92</a> —@JustinTrudeau

Ukrainian Foreign Minister Vadym Prystaiko said Iran was co-operating with an investigation, and that Kyiv was not yet ruling out any version of what might have happened.

Prystaiko was more guarded and said Ukraine did not want to come to conclusions yet about what caused the crash. However, Ukraine's state security chief, Ivan Bakanov, said he was prioritizing two versions of what might have caused the crash: a missile or terrorism.

Ukraine has previously said a collision or an engine explosion were other possible causes.

Prystaiko said the issue of where the black boxes would be analyzed was still being discussed, but that Ukraine wanted this to take place in Kyiv.

Canada and others have said the plane was brought down by an Iranian missile, probably by mistake. All 176 people on board the flight died in the crash of the Boeing 737-800. (Ebrahim Noroozi/THE ASSOCIATED PRESS)

Flight PS752 to Kyiv from Tehran crashed on Wednesday, when Iran was on alert for a possible U.S. military response to missiles it fired at U.S. targets in Iraq hours earlier.

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.

Iran denied the Boeing 737-800 was downed by a missile and rejected suggestions the crash site has been cleared by bulldozers. Prystaiko also said he could not confirm evidence of bulldozers being at the site.

Watch: Video of 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

He said there were many versions of what could have caused the crash. "We do not reject any of the versions," Prystaiko said at a televised briefing in Kyiv. "We want to establish the truth."

"At the moment, we have no reason to say that the Iranian authorities are not co-ordinating their activities or are not sufficiently interacting with Ukraine," he added.

Prystaiko said Ukrainian flights to Iran and Iraq were banned until the investigation has been concluded.

He also said the plane had changed course after an incident, but it was hard to say what the reason was.

Canadian investigators invited, but access limited

The Transportation Safety Board of Canada (TSB) said Thursday it will go to the crash site after being invited by Iran's Aircraft Accident Investigation Bureau. So far, Iran is offering Canada only limited access to its crash probe.

Global Affairs Canada (GAC) said Friday that, in total, they're seeking visas for two TSB investigators and 10 consular officials from GAC. So far, Iran has granted two visas for consular officials.

Champagne said the investigators and officials are currently stationed in the Turkish capital Ankara.

"We are hoping the other visas will be approved soon so that we may begin to provide consular services, to help in the identification of victims and to participate in any investigation," Champagne said in a tweet.

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 also reported that officials were inviting Boeing experts to join the investigation.

If the U.S. 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.

Watch: 'It's a missile strike,' says aviation expert

Aviation expert says the downed plane in Iran should not have been in the air in the first place 8:39

The Iranian public had rallied around the nation's leadership after the killing of Gen. Qassem 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 that were sparked by an economic crisis, which was exacerbated by U.S. 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 U.S. forces in 1988 — which killed all 290 people aboard — as proof of American hostility.