A Russian Ilyushin-76 military transport plane has crashed in the southern Belgorod region bordering Ukraine.
Russia’s ministry of defence said 65 captured Ukrainian military were on the plane heading to Belgorod region for a prisoner exchange.
None of the details surrounding those on board can be independently verified.
Ria Novosti news agency said another nine people were on the plane, including six crew.
Ukraine’s general staff, quoted by the Ukrainska Pravda website, said the plane was transporting missiles for Russia’s S-300 air defence systems. It made no mention of prisoners of war.
However, the head of Ukraine’s military intelligence directorate, Andriy Yusov, said a prisoner exchange had been due to take place.
“I can state that the exchange planned for today is not going ahead,” he told Radio Liberty.
Video shared on social media showed a plane going down followed by an explosion and a fireball near the village of Yablonovo, 70km (44 miles) to the north-east of the city of Belgorod, at around 11:00 local time (08:00 GMT).
The regional governor in Russia’s Belgorod region, Vyacheslav Gladkov, said the plane crashed in a field near a residential area and that everyone on board had died.
Some Ukrainian media suggested initially that the Il-76 may have been downed by Ukrainian forces, but those reports were later deleted.
Ukraine’s general staff told BBC Ukrainian that it did not have accurate information about the situation and that it was investigating the circumstances.
The Ukrainian government body in charge of prisoners of war warned that Russia was “actively carrying out special information operations against Ukraine, which are aimed at destabilising Ukrainian society”.
The crash took place on Russian territory and there is little chance of corroborating what happened.
Dmytro Lubinets, Ukraine’s parliamentary human rights commissioner, appealed to Ukrainians to trust only official sources: “Do not be fooled by provocations. More detailed information will be provided later.”
In a later statement, the Russian defence ministry said military transport plane had taken off from Chkalovsky air base north-east of Moscow en route for Belgorod, alleging that Ukraine’s air force had fired two anti-aircraft missiles from the Lyptsi area south of the Ukrainian border.
A prisoner exchange had been due to take place on Wednesday afternoon at a border checkpoint 100km to the west of Belgorod, it added.
Andrei Kartapolov, the chairman of Russia’s parliamentary defence committee, claimed there had been a second plane in the air transporting 80 Ukrainian prisoners, although that plane had then changed course.
“There can now be no talk of any other [prisoner] exchanges,” Mr Kartapolov told Russian TV.
Ukraine and Russia have taken part in a number of prisoner exchanges since the start of the war.
The biggest swap so far took place earlier this month, when Ukraine freed 248 Russian prisoners of war and Russia released 230 people in a deal mediated by the United Arab Emirates.
More than 8,000 Ukrainians, both civilian and military, are still being held by Russia, according to Ukraine’s Coordination HQ on Treatment of PoWs, which says tens of thousands of others are still missing.
A nationwide air raid alert was briefly in place across Ukraine shortly after news of the Il-76 crash emerged.
Belgorod, which is located approximately 25 miles (40km) north of the border with Ukraine, has suffered dozens of casualties from air strikes and drones since the war in Ukraine began.
In December, 25 people were killed and 100 were injured following an air strike – although Ukraine insisted that only military infrastructure had been targeted and blamed Russian air defences for fragments falling on the city.
Russia launched a full-scale invasion of Ukraine in February 2022 and the war is grinding on as it approaches its third year.
Last week, Russia claimed to have captured a village close to the devastated city of Bakhmut, in Ukraine’s eastern Donetsk region. Kyiv has not confirmed the claim.
Meanwhile, Russian air attacks have intensified in recent weeks. On Tuesday, 18 people were killed and 130 were injured in missile attacks on Ukrainian cities.
Ukrainian Defence Minister Rustem Umerov has warned that while Ukrainian forces are suffering from a shortage of ammunition, Russia has used more than 600 missiles and more than 1,000 drones in the past two months.
For its part, Ukraine is fighting mostly by using drones. At the weekend, a drone attack caused an explosion at a major gas export terminal near the city of St Petersburg.