Kyiv, Ukraine - Ukraine's SBU security and intelligence service on Thursday opened a criminal probe into the downing of a Russian military plane that Moscow said killed 65 Ukrainian prisoners of war.

"The security service of Ukraine has opened a criminal investigation into the downing of an IL-76 Russian Air Force plane in the Belgorod region," the SBU press service said in a statement.

Russia on Wednesday claimed 74 people died – 65 of them Ukrainian POWs being flown to a scheduled exchange – when a military transport plane was shot down in the western Belgorod region near the border with Ukraine.

Ukrainian President Volodymyr Zelensky had accused Russia of "playing with the lives of Ukrainian prisoners" while neither confirming nor denying his country's involvement in the incident.

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Admitting it had been a "very difficult day," he added: "We need to establish all the clear facts. As much as possible, given that the downing of the plane occurred on Russian territory, which is beyond our control."

Zelensky also called for an international investigation.

Russia immediately blamed Ukraine for shooting down the plane, calling it a "terrorist act."

"It is of course a monstrous act," Kremlin spokesperson Dmitry Peskov said, adding that it was too early to judge what kind of impact it would have on extending the prisoner exchange program.

In a series of carefully-worded statements, various military agencies in Kyiv confirmed an exchange had been scheduled to take place and said Russia was responsible for protecting Ukrainian POWs in its custody, while vowing to continue targeting Russian military facilities and planes in the Belgorod region.