"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. This may well have been unintentional," the prime minister said in a press conference in Ottawa on Thursday, Efe news reported.

The Iranian government responded to the Canadian prime minister's comments, with spokesman Ali Rabiei denying in a statement that the commercial airliner had been shot down.

"All these reports are a psychological warfare against Iran ... all those countries whose citizens were aboard the plane can send representatives and we urge Boeing to send its representative to join the process of investigating the black box," Rabiei said.

Trudeau said Canadian officials believe that the supposed shootdown of the airliner "may have been unintentional."

The Canadian prime minister refused to answer questions from reporters about whether Canada considered the United States the ultimate party responsible for the accident because of Washington's confrontation with Tehran after the killing of top Iranian Gen. Qasem Soleimani on Jan. 3 by a US drone at the Baghdad airport.

Trudeau said it was "too soon to be apportioning blame" for the crash or "drawing any conclusions" about what happened to the airliner.

"The families of the victims and all Canadians want answers. I want answers. That means closure, transparency, accountability and justice," Trudeau said. "This government will not rest until we get that."

UIA flight PS752, which was headed from Tehran to Kiev, was carrying 167 passengers and nine crew members.

Sixty-three Canadians were aboard the Boeing 737-800, which crashed just outside Tehran.

The plane was also carrying 82 Iranians; 11 Ukrainians, including two passengers and nine crew members; 10 Swedes; four Afghans; three Germans; and three Britons.

The Boeing 737-800 went down shortly after Iran fired dozens of missiles at bases in neighbouring Iraq used by the US military in retaliation for Soleimani's killing.

"It is now more important than ever that we know exactly how such a tragedy could have happened," Trudeau said.

The prime minister said his government asked Iran to permit Canadian investigators to participate in the crash investigation, but Tehran has not agreed to do so even though Iranian officials demonstrated an "openness" to the idea.

"Canadians have questions and they deserve answers," Trudeau said.

The prime minister held his press conference in the capital not long after US President Donald Trump said he had "suspicions" about what happened to the Boeing 737-800.

"It's a tragic thing when I see that, it's a tragic thing. But somebody could have made a mistake on the other side," the US president said.

Earlier in the day, Iranian Civil Aviation Organization director Ali Abedzadeh told the ISNA news agency that the plane was not shot down.

"Scientifically, it is impossible that a missile hit the Ukrainian plane, and such rumors are illogical," Abedzadeh said.

Abedzadeh said eyewitnesses saw the UIA plane "on fire" before it went down and the pilots did not make any emergency calls before attempting to return to the airport.

"Several domestic and foreign flights were flying in Iranian space at the same altitude of 8,000 feet (2,400 meters). The issue of the missile's impact on the aircraft cannot be true in any way," Abedzadeh said.

The Boeing 737-800 took off early Wednesday and crashed minutes into its flight.

Canada is home to more than 250,000 people of Iranian descent. Many of the Canadian victims of the UIA crash were families and students who had traveled to Iran for the holidays.