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.
Two people were also killed in a Russian rocket strike in Ukraine's eastern Donetsk region, local authorities said.
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".
"We currently do not have reliable or comprehensive information on who was on board the plane or in what number," Ukraine's main intelligence agency said in a statement.
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 the planned prisoner exchange 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.
Ukraine's intelligence agency denied this and accused Russia of "endangering the lives and safety of prisoners".
"The Ukrainian side was not informed about the need to ensure the security of the airspace in the area around the city of Belgorod at a set time, as has been done many times in the past," the intelligence directorate said.
- Muted reaction in Kyiv -
The threat facing Ukraine was highlighted later on Wednesday when two people, including a 16-year-old, were killed and eight wounded when a Russian rocket hit the village of Girnyk, Donetsk regional head Vadim Filachkine said, accusing Moscow of targeting civilians.
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.
It said that a number of shelling attacks on Ukrainian territory were "directly related" to Russian military transport aircraft flying to Belgorod airfield.
"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," she told the RIA-Novosti news agency. "There was already smoke, fire and rumbling."
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 carried out 49 prisoner exchanges since the conflict began almost two years ago.
Kyiv 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.
In Russia, the plane carrying the leader of the Wagner mercenary group, Yevgeny Prigozhin, crashed last August on a flight from Moscow to St Petersburg.
Moscow denied involvement, claiming instead that the plane crashed because its passengers detonated a grenade on board.
Meanwhile, Western military alliance NATO on Wednesday announced that it had begun its biggest military exercise since the Cold War, which will last months and involve 90,000 troops.