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Iran has admitted it unintentionally shot down a Ukrainian passenger plane hours after launching ballistic missiles at Iraqi bases hosting U.S. troops, blaming "human error" for the "great tragedy" that killed all 176 people aboard.

"Armed Forces’ internal investigation has concluded that regrettably missiles fired due to human error caused the horrific crash of the Ukrainian plane & death of 176 innocent people. Investigations continue to identify & prosecute this great tragedy & unforgivable mistake," Iranian President Hassan Rouhani said on Twitter.

Based on a preliminary conclusion by the armed forces, "human error at time of crisis caused by US adventurism led to disaster," Iranian Foreign Minister Mohammed Javad Zarif tweeted, seemingly attempting to lay some blame for the incident on the United States.

The country's Islamic Revolutionary Guard Corps (IRGC) said in a statement that the military had been on high alert after launching the strikes in Iraq as retaliation for the U.S. killing of top general Qassem Soleimani.

The Ukraine International Airlines Flight PS752 was shot down just hours later, mistaken for a cruise missile.

The IRGC took full responsibility for the tragedy. In an address broadcast by state TV, aerospace division head Gen. Amir Ali Hajizadeh said that when he learned about the downing of the plane, "I wished I was dead."

Ukrainian President Volodymyr Zelenskiy said in a statement Saturday that the investigation must continue.

"Iran has pleaded guilty to shooting down the Ukrainian plane. But we insist on the full admission of guilt," Zelenskiy said.

"We expect Iran to assure the readiness for a full and open investigation, bringing those responsible to justice, the return of the bodies of the victims, payment of compensation, official apologies through diplomatic channels."

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At a briefing for reporters, Ukraine International Airlines said it received no warning about a possible threat, adding that Iran should have closed the airspace.

Iran had previously denied shooting down the flight shortly after it took off from Tehran early Wednesday.

Prior to Iran's admission Saturday, U.S. intelligence officials had evidence that the plane was shot down by an Iranian missile by mistake, multiple officials said.

Canadian Prime Minister Justin Trudeau said that his country also had evidence that the plane was shot down.

In a statement tweeted by his communications director after Iran's admission, Trudeau called for transparency.

“Our focus remains closure, accountability, transparency, and justice for the families and loved ones of the victims. This is a national tragedy, and all Canadians are mourning together,” Trudeau said, adding that he expects full cooperation from Iranian authorities.

Those aboard the doomed flight included 82 Iranians, 63 Canadians and 11 Ukrainians, including the nine crew members. Ten were from Sweden, four from Afghanistan, three from Germany and three from the United Kingdom.

Iran expressed condolences to the families of all of those killed, and Saturday's military statement suggested those responsible would be referred for criminal charges.

The country's supreme leader, Ayatollah Ali Khamenei, expressed his “deep sympathy” to the families of the 176 victims and called on the armed forces to "pursue probable shortcomings and guilt in the painful incident.”

The Boeing 737-800 was headed from Tehran to Kyiv, Ukraine, where more than 130 were to change planes to head to Canada, Trudeau has said.

Among those killed was a 9-year-old girl who died with her mother, husband and wife professors, and graduate students. The University of Alberta alone lost 10 people, including professors, students and alumni.

Similar tragedies have struck in the past.

In 2014, a passenger plane, Malaysia Airlines Flight MH17, was shot down amid military conflict in Ukraine, killing all 298 people aboard.

A Dutch-led team of investigators and prosecutors two years later concluded it was shot down by a surface-to-air missile fired from a Russian launcher in an area of Ukraine controlled by pro-Moscow rebels.

An international team of investigators in 2018 said that the Buk missile came from a Russia-based military unit. Russia has denied shooting down the plane.

In 1988, a U.S. Navy guided-missile cruiser mistakenly shot down an Iranian passenger plane, Iran Air Flight 655, killing all 274 passengers and the 16 crew aboard.

Last week, after President Donald Trump threatened to target 52 sites in Iran should leaders there retaliate for the death of Soleimani, Iran's president appeared to make reference to the 1988 incident.

"Those who refer to the number 52 should also remember the number 290. #IR655,” Rouhani said in a tweet, warning "never threaten the Iranian nation."