A leaked recording of an exchange between an Iranian air-traffic controller and an Iranian pilot purports to show that authorities immediately knew a missile had downed a Ukrainian jetliner after takeoff from Tehran, killing all 176 people aboard — 57 of them Canadian — despite days of denials by the Islamic Republic.

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

In Tehran on Monday, the head of the Iranian investigation team, Hassan Rezaeifar, acknowledged the recording was legitimate and said that it was handed over to Ukrainian officials.

After the Jan. 8 disaster, Iran's civilian government maintained for days that it didn't know the country's paramilitary Revolutionary Guard, answerable only to Supreme Leader Ayatollah Ali Khamenei, had shot down the aircraft. The downing of the jetliner came just hours after the Guard launched a ballistic missile attack on Iraqi bases housing U.S. forces in retaliation for an earlier American drone strike that killed the Guard's top general, Qassem Soleimani, in Baghdad.

WATCH | Leaked recording could further delay Flight 752 investigation:

A leaked recording between an Iranian pilot and air traffic control could further delay investigations into the crash of Flight 752. 1:56

A transcript of the recording, published by Ukrainian 1+1 TV channel, contains a conversation in Farsi between an air-traffic controller and a pilot reportedly flying a Fokker 100 jet for Iran's Aseman Airlines from Iran's southern city of Shiraz to 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 the Payam airport, near where the Guard'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.

Fifty-seven Canadians were killed when Ukrainian International Airlines Flight 752 was shot down near Tehran on Jan. 8, killing all 176 aboard. Above, mourners attend a memorial for the victims at Imam Mahdi Islamic Centre in Thornhill, Ont. Jan. 18. (Carlos Ororio/Reuters)

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

Publicly accessible flight-tracking radar information suggests the Aseman Airlines aircraft, flight No. 3768, was close enough to Tehran to see the blast.

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

Iran condemns publication of recordings

Iranian officials should have immediately had access to the air-traffic control recordings and Zelenskiy told 1+1 that "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."

Ukraine's president 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 so-called "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, the head of the Iranian investigators, according to a report by the semiofficial Mehr news agency.