Russia on Wednesday accused Ukraine of shooting down a military transport plane carrying dozens of Ukrainian prisoners headed for a prisoner exchange, killing everyone on board.
Kyiv confirmed that a prisoner exchange was due to take place on Wednesday, but said it had no reliable information on the passengers of the downed plane.
Videos on social media showed a large plane 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".
Ukraine's main intelligence agency said in a statement that "a prisoner exchange was supposed to take place today, but it did not take place".
"According to the Russian side, this was due to the downing of a Russian Il-76 aircraft, which was allegedly transporting our prisoners. We currently do not have reliable or comprehensive information on who was on board the plane or in what number," it said.
A Ukrainian government body responsible for issues linked to prisoners of war said earlier that it was probing Russia's 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 Russian 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 the Belgorod aerodrome today," it said.
Local Ukrainian 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.
In a carefully worded statement published hours after the crash, the Ukrainian army said it would continue to target Russian aircraft in Belgorod region.
But the statement did not mention Wednesday's crash, or address Moscow's accusations.
It said that a number of shelling attacks on Ukrainian territory were "directly related" to Russian military transport aircraft flying to Belgorod airfield.
"With this in mind, the Ukrainian army will continue to take measures to destroy delivery vehicles and control the airspace to eliminate the terrorist threat, including in the Belgorod-Kharkiv direction," it said.
Ukrainian rights ombudsman Dmytro Lubinets, who is among officials responsible for prisoner exchanges, said his office was investigating the incident.
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.
"The plane crashed far away from the village. It was very loud and very scary," said Maria Mexentseva, a resident of the nearby village of Yablonovo.
"We just heard a loud rumble. We went outside. There was already smoke, fire and rumbling, that's it," she told the RIA-Novosti news agency.
Kremlin spokesman Dmitry Peskov declined to comment on the crash, saying news 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".
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.
Moscow denied involvement, claiming instead that the plane crashed because its passengers detonated a grenade on board.
In October 2022, a 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.
