Korochansky, Russia - Russia on Wednesday accused Ukraine of shooting down a military transport plane carrying dozens of Ukrainian prisoners, killing everyone on board.

Unverified videos on social media showed a large plane, purportedly in Russia's western Belgorod region, falling from the sky on its side before crashing in a fireball.

Russia's defense ministry said the IL-76 plane was carrying 65 Ukrainian soldiers captured in Russia's offensive, six crew, and three escorts.

It claimed Ukrainian forces stationed in the Kharkiv border region had fired two missiles at the transport aircraft and described the incident as a "terrorist act."

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A Ukrainian government body responsible for issues linked to prisoners of war said it was probing the Russian claims.

AFP could not independently verify the Russian claims, but the incident would represent one of the single most deadly episodes of the full-scale conflict in weeks.

The defense ministry said the crash had come just hours ahead of an agreed prisoner exchange later on Wednesday that was set to take place at a border crossing in Belgorod.

"The Ukrainian leadership was well aware that, in accordance with established practice, the Ukrainian servicemen to be exchanged would be transported by military transport aircraft to Belgorod aerodrome today," it said.

The reaction in Kyiv was muted.

Local media initially cited defense sources saying that the Ukrainian army had downed the plane, and that it was carrying missiles. The claim was later retracted.

Ukraine's intelligence agency said Wednesday it had no "reliable information" on who was on board the plane, but acknowledged that a prisoner swap was supposes to take place.

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Ukrainian rights ombudsman Dmytro Lubinets, who is among officials responsible for prisoner exchanges, said his office was investigating.

The crash occurred in the Korochansky district, northeast of the Belgorod region's capital at around 3:00 AM ET, governor Vyacheslav Gladkov said.

He added that the aircraft was downed in a field and the area had been closed off while investigators and emergency services worked at the scene.

The head of Russian-state broadcaster RT published a list of captured Ukrainian servicemen allegedly on board.

The issue of prisoners of war is sensitive in both countries. Despite full-scale hostilities, the two sides have managed to carry out 49 prisoner exchanges since the conflict began almost two years ago.

Ukraine says more than 8,000 Ukrainians remain in Russian captivity, including civilians.