A Russian Ilyushin-76 military transport plane has crashed in the southern Belgorod region bordering Ukraine.
At least 65 Ukrainian prisoners of war were killed, according to Russia's ministry of defence.
Ria Novosti news agency said another nine people were on board, including six crew.
None of the details surrounding those on board can be independently verified.
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.
Some Ukrainian media suggested initially that the Il-76 may have been downed by Ukrainian forces, but those reports were later deleted.
Belgorod regional governor Vyacheslav Gladkov said he was aware of an "incident" but gave no further details.
Video shared on social media showed a plane going down followed by an explosion near the village of Yablonovo, 70km (44 miles) to the north-east of Belgorod, at around 11:00 local time (08:00 GMT).
According to Ria Novosti, the Ukrainian prisoners were "being transported to Belgorod region for exchange", although that cannot be confirmed.
Russian presidential spokesman Dmitry Peskov said the Kremlin was aware of the crash but refused to go in to details.
