A leaked recording of an exchange between an Iranian air-traffic controller and an Iranian pilot purports to show that Iranian authorities immediately knew its missiles had downed a Ukrainian plane after takeoff from Tehran, killing all 176 people on board, despite days of denials.

Ukraine's President Volodymyr Zelenskyy acknowledged the recording's authenticity in a report aired by a Ukrainian television channel on Sunday night.

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Hassan Rezaeifar, head of Iran's investigation team, acknowledged the recording was legitimate and said it was handed over to the Ukrainian officials in Tehran on Monday.

After the January 8 disaster, Iran's civilian government maintained for days that it did not know Iran's Islamic Revolutionary Guard Corps (IRGC), answerable only to Supreme Leader Ayatollah Ali Khamenei, had shot down the aircraft.

Leaked conversation

A transcript of the recording, published by Ukrainian 1+1 TV channel, shows a conversation in Farsi between the air-traffic controller and a pilot reportedly flying a Fokker 100 jet for Iran's Aseman Airlines from the southern Iranian city of Shiraz to capital Tehran.

"A series of lights like ... yes, it is missile, is there something?" the pilot calls out to the controller.

"No, how many miles? Where?" the controller asks.

The pilot responds that he saw the light by Payam airport, near where the IRGC's Tor M-1 anti-aircraft missile was launched from.

The controller says nothing has been reported to them, but the pilot remains insistent.

"It is the light of missile," the pilot says.

"Don't you see anything anymore?" the controller asks.

"Dear engineer, it was an explosion. We saw a very big light there, I don't really know what it was," the pilot responds.

The controller then tries to contract the Ukrainian aircraft, but unsuccessfully.

Publicly accessible flight-tracking radar information suggests Aseman Airlines flight number 3768 was close enough to Tehran to see the blast.

Iranian civil aviation authorities for days insisted it was not a missile that brought down the plane, even after Canadian Prime Minister Justin Trudeau and US officials began saying they believed it had been shot down.

Zelenskyy told 1+1: "The recording, indeed, shows that the Iranian side knew from the start that our plane was shot down by a missile, they were aware of this at the moment of the shooting."

Black boxes

The Ukrainian president also repeated his demands to decode the plane's flight recorders in Kyiv - something Iranian officials had promised last month but later backtracked on.

On Monday, Ukrainian investigators were to travel to Tehran to participate in the decoding effort, but Zelenskiy insisted on bringing the "black boxes" back to Kyiv.

"It is very important for us," he said.

Iranian authorities, however, condemned the publication of the recording as "unprofessional", saying it was part of a confidential report.

"This action by the Ukrainians makes us not want to give them any more evidence," said Rezaifar, head Iranian investigator, according to a report by the semi-official Mehr news agency.

Al Jazeera's Assed Baig, reporting from Tehran, said the revelation puts Tehran in "a very difficult position".

"This has upset the Iranians, not only because this information has been leaked without anyone telling them, but also because domestically and politically it puts them in a difficult position," he said.