Prime Minister Justin Trudeau says he told Iranian President Hassan Rouhani on Saturday that Tehran must take full responsibility for downing Ukrainian International Airlines flight PS752 and killing all 176 passengers and crew on board, including 57 Canadians.

Trudeau said he is both "outraged" and "furious" over the incident and expects full co-operation from Iranian authorities in investigating the circumstances that led to the crash.

"We need full clarity on how such a horrific tragedy could have occurred," Trudeau said at a press conference on Saturday. "Families are seeking justice and accountability and they deserve closure."

The plane crashed Wednesday after it was struck by missiles launched by the Iranian Revolutionary Guard. It crashed just minutes after it took off from Tehran's Imam Khomeini Airport, and only hours after Iran had launched a ballistic missile strike on two military bases housing U.S. and Canadian troops in retaliation for the assassination of Gen. Qassem Soleimani.

Rouhani said Saturday that an investigation by the Iranian military found that the missiles were fired due to "human error."

A military statement carried by state media said the plane was mistaken for a "hostile target" after they claim it turned toward a "sensitive military centre" of Iran's Revolutionary Guard.

The statement from the Iranian government marked a significant reversal from the Iranians. Only two days earlier, an official with the country's Civil Aviation Organization was quoted as saying it was "impossible" that a missile hit the Ukrainian plane, in response to a news conference in which Trudeau said that intelligence indicated the plane was shot down by an Iranian missile.

The Islamic Republic of Iran deeply regrets this disastrous mistake.<br><br>My thoughts and prayers go to all the mourning families. I offer my sincerest condolences. <a href="https://t.co/4dkePxupzm">https://t.co/4dkePxupzm</a> —@HassanRouhani

Admission good first step, Trudeau says

Trudeau said Iran's admission was an important step in providing answers for families, but that more must be done.

"I reiterated to President Rouhani that it is absolutely necessary that Canada participate in this investigation," Trudeau said. "We expect the full co-operation of Iranian authorities."

Trudeau was joined by Ukrainian President Volodymyr Zelensky in calling for a thorough investigation of the causes.

"We expect Iran to bring those responsible to justice, return the bodies, pay compensation and issue an official apology," Zelensky said in a tweet. "The investigation must be full, open and continue without delays or obstacles."

Trudeau, along with Foreign Affairs Minister François-Philippe Champagne, will attend a vigil for victims of the crash on Sunday in Edmonton. The prime minister is expected to deliver remarks at the event, according to a news release from the Prime Minister's Office. CBC News will provide live coverage of the event beginning at 5 p.m. ET.

Trudeau said Rouhani promised further investigation into the circumstances surrounding the plane crash.

"His response to me was a commitment to collaborate, to give closure to the victims, to de-escalate tensions in the region and continue this dialogue,"​​ said Trudeau.

Watch: Trudeau's statement Saturday following Tehran's admission it shot down plane

Prime Minister Justin Trudeau held a news conference in Ottawa on Saturday, saying Iran must take "full responsibility" and that a full and complete investigation must be conducted. 6:12

The phone call marks what appears to be the first time Canadian and Iranian heads of government have spoken in years, and underscores the gravity of the diplomatic crisis that has emerged in the days since the UIA flight PS752 went down.

Canada and Iran cut diplomatic ties in 2012 when then-Prime Minister Stephen Harper expelled all Iranian diplomats from the country.

Four Conservative MPs— Erin O'Toole, James Bezan, Pierre Paul-Hus, and Todd Doherty — made several demands on the government, including listing Iran's Revolutionary Guard as a terrorist organization, demanding full compensation for victims' and considering new sanctions on Iranian officials under the Magnitsky Act.

"The fact that 57 Canadians lost their lives due to an Iranian missile requires action by the Trudeau Liberals," the statement said. "The status quo is unacceptable."

Visas for Canadian officials

Trudeau said he is somewhat encouraged by initial signs of Iranian co-operation in allowing Canadian officials access to the country.

Iran has approved visas for three members of Canada's Rapid Deployment Team, Trudeau said, who are expected to arrive in Iran at 4 p.m. ET Saturday to provide consular support to Canadians whose family members were killed in the flight.

Seven other consular officials and two investigators from the Transportation Safety Board are currently in Ankara, Turkey, and are waiting for visas.

Trudeau said he expects more visas to be granted in the coming days.

Immigration Minister Marco Mendocino said his department has set up a hotline that family members can call to help with visa issues.

In response to the Ukrainian International Airlines tragedy, <a href="https://twitter.com/CitImmCanada?ref_src=twsrc%5Etfw">@CitImmCanada</a> is putting in place special measures for the families of the victims of Flight 752. Starting today, families may call 1-833-864-2831 to address immigration issues stemming from the tragedy. —@marcomendicino

Regional tensions

Roland Paris, professor of international affairs at the University of Ottawa, said the Trudeau government's "careful and methodical" approach to Iran in the days following the crash paid off.

"Not immediately attacking Iran for a cover-up may have allowed and given some space to the Iranian government to make what was for them, I'm sure, a difficult decision," said Paris.

By admitting its role in the incident, Paris said, the Iranian government opened the door to public anger in the country.

Indeed, protestors took to the streets of Tehran today to demand that officials involved in the missile attack be brought to justice. Read more about the protests in Iran here.