Russia said Wednesday that an IL-76 military transport plane carrying 65 Ukrainian prisoners had crashed in the western Belgorod region, bordering Ukraine, killing everyone on board.
Unverified videos on social media showed a large plane, purportedly in the Belgorod region, falling from the sky on its side before crashing in a fireball.
Moscow's defence ministry said the plane had crashed at around 0800 GMT "during a routine flight".
"On board were 65 captured Ukrainian army servicemen being transported to the Belgorod region for exchange, six crew members and three escorts," it added.
Vyacheslav Volodin, the top lawmaker in Russia's lower house of parliament, claimed the plane had been shot down by Kyiv and blamed Western missiles.
"They shot their own soldiers in the air. Their own," Volodin told lawmakers in a plenary session. "Our pilots, who were carrying out a humanitarian mission, were shot down."
The crash occurred in the Korochansky district, northeast of the region's capital, regional governor Vyacheslav Gladkov said on social media.
"Everyone on board died," he wrote in a later post.
He said the plane had crash in a field and the area had been closed off while investigators and emergency services worked at the scene.
AFP was not able to immediately verify Russia's claims.
In Kyiv, local media initially cited sources in the Ukrainian security services as saying that its army had downed the plane, and that it was transporting missiles. But media outlets later retracted the claim.
Ukrainian rights ombudsman Dmytro Lubinets, who is among officials responsible for prison exchanges, later said his office was probing the incident.
Kremlin spokesman Dmitri Peskov declined to comment, saying news around the crash was still emerging but said Russian authorities would "look into" the incident.
The issue of prisoners of war is sensitive in both countries.
In 2022, Russia and Ukraine accused each other of carrying out deadly bombardments on a jail holding dozens of captured Ukrainian servicemen in Kremlin-controlled Olenivka, in eastern Ukraine's Donetsk region.
Both Moscow and Kyiv blamed each other for the incident, which President Volodymyr Zelensky called a "Russian war crime".
- Strikes on Ukraine -
Since launching large-scale hostilities in Ukraine in February 2022, several Russian military aircraft have crashed, and Ukraine has also claimed to have shot down Russian war planes.
Last week, Kyiv claimed to have downed an A-50 Russian reconnaissance plane and damaged an Il-22 bomber over the Azov Sea, lying between the two countries.
In Russia, the plane carrying the leader of the Wagner mercenary group, Yevgeny Prigozhin, crashed last August on a flight from Moscow to Saint Petersburg.
Prigozhin died alongside his top aides in the incident, which came two months after they attempted to topple Russia's military leadership, angering President Vladimir Putin.
Moscow denied involvement, claiming instead that the plane crashed because its passengers detonated a grenade on board.
In October 2022, an Su-34 fighter bomber flew into a residential building in the town of Yeysk, on the Russian coast of the Sea of Azov, leaving more than a dozen dead.
Both Russian and Ukraine have stepped up strikes on each others' cities and critical infrastructure in recent weeks.
Ukrainian President Volodymyr Zelensky said that at least 18 people had been killed and more than one hundred others were wounded in a Russian missile barrage early Tuesday.
Kyiv has urged its allies to help bolster its air defence systems to ward off Russian attacks.