Prime Minister Justin Trudeau says most of the passengers on the Ukraine International Airlines flight that crashed in Iran this morning, killing everyone aboard, were destined for Canada.

"At least 63 Canadians were on board and a total of 138 passengers on that flight were connecting to Canada — all people who won't be coming home to their parents, their friends, colleagues or their family," Trudeau told a press briefing in Ottawa this afternoon, hours after 176 people on Flight PS752 from Tehran to Kyiv — including Canadian families, academics and students — were killed.

"While no words will erase your pain, I want you to know that an entire country is with you. We share your grief."

Trudeau repeated his promise to make certain the crash — one of the deadliest disasters involving Canadian citizens in decades — is thoroughly investigated. He said Foreign Affairs Minister François-Philippe Champagne will soon speak with his Iranian counterpart to drive home that point.

"Canadians have questions and they deserve answers," he said, flanked by Defence Minister Harjit Sajjan, Transport Minister Marc Garneau and Chief of the Defence Staff Gen. Jonathan Vance.

A Ukraine International Airlines worker reacts during a briefing at Borispil International Airport outside Kyiv on Wednesday. A Ukrainian airplane carrying 176 people crashed on Wednesday shortly after takeoff from Tehran's main airport, killing all aboard. Ukraine's foreign minister says 63 Canadians are among the dead. (Efrem Lukatsky/The Associated Press)

Neither Trudeau nor Garneau would speculate on the cause of the incident.

Trudeau was asked if he could categorically say that the crash, which happened just hours after Iran launched a ballistic missile attack on Iraqi bases housing U.S. soldiers, was not shot down.

"I cannot. It is too early to speculate," he said.

At first, both Ukrainian and Iranian officials said they suspected a mechanical issue brought down the Boeing 737-800 aircraft, but the Ukrainian Embassy in Iran later said any previous comments about the cause of the crash were not official.

'Something very unusual happened': Garneau

Garneau, a former astronaut, says satellite data suggest the Boeing 737-800 aircraft took off normally, but officials lost contact with the flight almost immediately after.

Transport Minister Marc Garneau says that Canada has offered to help with the investigation of the crash that claimed the lives of at least 63 Canadian victims. 0:56

"We lost contact with it, suggesting that something very unusual happened, but we cannot speculate at this point," he said.

"There are a number of possibilities and we will have to wait to obtain more information, perhaps from the black boxes or from other intelligence, but at this point it's too early to speculate."

Canada's Transportation Safety Board said Iranian officials will lead the investigation and it will appoint a Canadian expert to receive and review information from the probe.

"The TSB remains available to provide any technical assistance requested by Iranian and Ukrainian accident investigation bodies," says the agency's statement.

Prime Minister Justin Trudeau says it is too early to speculate on the cause behind the crash of the Ukraine Airlines flight that killed at least 63 Canadians. 0:35

Ukraine International Airlines, which has suspended flights to Tehran indefinitely, issued a statement saying the aircraft was built in 2016 and underwent its last scheduled maintenance Monday.

The work of probing the cause of the crash and repatriating the bodies of the Canadians will be complicated by the fact that Canada severed diplomatic ties with Iran years back.

"I think all sorts of issues will be hampered and, on top of that, it prevents Canada from mediating in this difficult situation that we're in right now," said Pouyan Tabasinejad, vice-president of the Iranian Canadian Congress.

"The repatriation will take more time. Getting information and working with civilian aviation authorities in Iran is hampered."

Situation evolving: foreign minister

Champagne has called the situation "extremely fluid" and said the number of Canadians confirmed dead could change as more information becomes available, including news on dual citizens.

"I have been in touch with my Ukrainian counterpart and will continue to speak to all relevant authorities," he said in a media statement.

"We will continue to keep Canadians informed as the situation evolves."

At least 27 people from Edmonton are believed to be among the dead. There are also reports of victims who lived in Nova Scotia, British Columbia, Quebec and Ontario.

The Peace Tower on Parliament Hill in Ottawa is shown with the Canadian flag at half mast to honour the victims of flight PS752 from Tehran to Kyiv which went down with 176 passengers and crew on board including 63 Canadians. (Marc Robichaud/CBC)

"We lost a significant portion of our community, and everybody in Edmonton that's of Iranian descent will know somebody that was on that flight," said Payman Parseyan, a member of the Iranian-Canadian community in the Alberta provincial capital.

Ukrainian Foreign Affairs Minister Vadym Prystaiko said Iranian, Ukrainian, Swedish, Afghan, British and German nationals were also aboard.

Read more about the known victims:

Ukraine's ambassador to Canada, Andriy Shevchenko, said his country is working with Iranian authorities on the ground to identify the bodies and help the families.

"My heart is broken," he said. "We will have to go through this terrible pain together with our Canadian brothers and sisters."

Debris is seen from an Ukrainian plane which crashed as authorities work at the scene in Shahedshahr, southwest of the capital Tehran, Iran, Wednesday, Jan. 8, 2020. (Ebrahim Noroozi/Associated Press)

Trudeau said he spoke with number of leaders, including U.S. President Donald Trump, French President Emmanuel Macron and U.K. Prime Minister Boris Johnson, who offered their condolences.

So did domestic party leaders and premiers.

NDP Leader Jagmeet Singh said the victims' families "deserve clear answers."

"But whatever the cause, this is devastating," he said. "Love to their families, friends and communities, and to everyone touched by this tragedy."

Conservative Leader Andrew Scheer called it the crash a "horrific tragedy," while Assembly of First Nations Chief Perry Bellegarde tweeted a reminder that "life is precious."

"Today is a sad day for our country," said Scheer.

Travel advisory for Canadians

Global Affairs Canada says friends and relatives of Canadian citizens believed to be on board can contact Global Affairs' emergency watch and response centre. Canadian citizens in Iran requiring consular assistance should contact the Canadian Embassy in Ankara or call Global Affairs Canada's emergency watch and response centre.

Canada is urging Canadians to avoid non-essential travel to Iran "due to the volatile security situation [and] the regional threat of terrorism."

"Canadians, particularly dual Canadian-Iranian citizens, are at risk of being arbitrarily questioned, arrested and detained," the warning adds.

Flowers and a memorial poster are placed outside the Iranian Embassy to commemorate the victims of the Ukraine International Airlines flight PS752 plane crash, in Kyiv, Ukraine January 8, 2020. The words on the poster read: 'Tehran-Kiev. We mourn.' (Valentyn Ogirenko/Reuters)

"Iran does not recognize dual nationality and Canada will not be granted consular access to dual Canadian-Iranian citizens. Canadian-Iranian dual citizens should carefully consider the risks of travelling to Iran."

The travel advisory does not mention the plane crash.

Trudeau on U.S. action against Iran

During his news conference on Wednesday, Trudeau confirmed that Canadian military personnel were present at an air base in Iraq hit by Iranian missile strikes Tuesday.

"I can confirm that there were Canadians at the ... base in Erbil when the rockets landed but they are all safe," he said.

When asked if he supported Trump's move to kill top Iranian military commander Qassem Soleimani with a drone strike last week, Trudeau said the decision was one for the U.S. to make. He did not say definitively whether he supports Trump's actions.

"Canada has been aware of the threat posed by the IRGC on regional and local safety and security — that is something we have long been aware of. The Americans made a decision based on their threat assessment," Trudeau said.

After Tuesday's missile attack, the U.S. Federal Aviation Administration issued a notice outlining flight restrictions prohibiting U.S. civil aviation operators from flying in the airspace over Iraq, Iran and the waters of the Persian Gulf and the Gulf of Oman.

There are no words.<br><br>176 lives lost.<br><br>63 Canadians won’t be coming home.<br><br>These families deserve clear answers, but whatever the cause, this is devastating.<br><br>Love to their families, friends, and communities and to everyone touched by this tragedy. <a href="https://t.co/jPSouzcFNZ">https://t.co/jPSouzcFNZ</a> —@theJagmeetSingh

Transport Canada followed up, tweeting that Air Canada — the only Canadian air carrier that operates in the region covered by the U.S. notice — has changed its routes in the region.

A spokesperson for the airline said it's rerouting flights to Dubai and will make further adjustments as needed.

"As a result of the current uncertain situation in the Middle East, like many international carriers Air Canada has taken precautionary measures," reads Air Canada's statement.

"Air Canada has not used Iranian airspace since mid-last year. These latest adjustments relate to Iraq airspace, which we will now also avoid."

WATCH | A former TSB investigator describes how the crash probe might unfold: