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 defence 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".
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 defence 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 defence sources saying that the Ukrainian army had downed the plane, and that it was carrying missiles. The claim was later retracted.
- Muted reaction in Kyiv -
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 0800 GMT, 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.
Kremlin spokesman Dmitry Peskov declined to comment, saying news around the crash was still emerging and that authorities would "look into" the incident.
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.
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".
- Crashed Russian planes -
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, flying 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 Russia and Ukraine have stepped up strikes on each others' cities and critical infrastructure in recent weeks.
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.