Iran says its military "unintentionally" shot down a Ukrainian passenger jet, killing all 176 people on board, Iran's state TV reports.
The statement said it had done so due to "human error" after the plane flew close to a sensitive site belonging to Iran's Revolutionary Guards.
Mistaken for a "hostile target", the jet was hit by a missile, the statement said.
Those responsible would be held accountable, it added.
Iran had previously rejected suggestions that one of its missiles brought down the plane near the capital, Tehran, on Wednesday.
But pressure mounted after the US and Canada, citing intelligence, said they believed Iran had shot down the plane with a missile, possibly accidently.
The downing of Ukraine International Airlines flight PS752 with the loss of 176 lives came just hours after Iran carried out missile strikes on two airbases housing US forces in Iraq.
The Ukrainian flight, en route to Kyiv, came down near Imam Khomeini Airport shortly after take-off.
US media had speculated that the Ukrainian airliner may have been mistaken for a warplane as Iran prepared for possible US retaliation.
The Iranian air strikes were launched in retaliation to the killing of top Iranian general Qasem Soleimani in a US drone attack on 3 January.
In a tweet on Saturday, Iranian Foreign Minister Mohammad Javad Zarif suggested "US adventurism" was partly to blame for the downing of the Ukrainian jet.
Victims of the disaster included 82 Iranians, 57 Canadians and 11 Ukrainians as well as nationals from Sweden, the UK, Afghanistan and Germany.
What does Iran's statement say?
On Saturday morning, an Iranian military statement read on state TV announced that it had struck flight PS752 with a missile by mistake.
The plane, the statement said, had flown near a "sensitive military centre" of the Revolutionary Guards, a force set up to defend the country's Islamic system.
Because of heightened tensions with the US, Iran's military "was at its highest level of readiness", the statement said. "In such a condition, because of human error and in an unintentional way, the flight was hit," it added.
In the statement, the military apologised for downing the plane, saying it would upgrade its systems to prevent such "mistakes" in the future.
Those responsible for firing the missile, it said, would be prosecuted.
What happened before Iran's announcement?
The announcement by Iran's military marks a stark departure from the denials of recent days. As recently as Friday, Iran was insistent that the plane had not been shot down.
"The thing that is clear to us and that we can say with certainty is that this plane was not hit by a missile," Iran's Civil Aviation Organisation (CAOI) chief Ali Abedzadeh said.
On Thursday, government spokesman Ali Rabiei accused the US and its allies of "lying and engaging in psychological warfare" in their speculation over the cause of the accident.
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