A Russian Ilyushin-76 military transport plane crashed in the southern Belgorod district, which borders Ukraine.
According to Russia's Ministry of Defence, 65 seized Ukrainian military personnel were on the plane bound for the Belgorod region for a prisoner swap.
None of the information about persons on board can be independently verified.
According to Ria Novosti, there were additional nine persons on the plane, including six crew members.
According to Ukraine's general staff, the jet was transporting missiles for Russia's S-300 air defense systems. It made no mention of POWs.
A plane crashed in the town of Yablonovo, 70 kilometers (44 miles) north-east of Belgorod, at around 11:00 local time (08:00 GMT), according to a video uploaded on social media.
Vyacheslav Gladkov, the regional governor of Russia's Belgorod region, reported that the jet crashed in a field near a residential neighborhood, killing everyone on board.
Some Ukrainian media outlets first stated that the Il-76 had been shot down by Ukrainian forces, however those stories were eventually removed.
Ukraine's general staff told BBC Ukrainian that they did not have proper information about the scenario and were looking into the facts.
The Ukrainian government authority in care of prisoners of war has warned that Russia is "actively carrying out special information operations against Ukraine, which are aimed at destabilising Ukrainian society".
Andrei Kartapolov, the chairman of Russia's parliamentary defense committee, later stated that a second jet was in the air, delivering 80 Ukrainian captives, but that plane had abruptly changed course.
"There can now be no talk of any other [prisoner] exchanges," Mr Kartapolov said on Russian television.
Dmitry Peskov, the Russian presidential spokesperson, said the Kremlin was aware of the disaster but declined to elaborate.
Following the announcement of the Il-76 accident, Ukraine momentarily issued a nationwide air raid alert.
Belgorod, located about 25 miles (40 kilometers) north of Ukraine's border, has lost hundreds of people to air strikes and drones since the war began.
In December, 25 civilians were killed and 100 injured in an air strike, despite Ukraine's claim that only military facilities was targeted and blaming Russian air defenses for fragments falling on the city.
Russia started a full-scale invasion of Ukraine in February 2022, and the conflict is approaching its third year.
Last week, Russia claimed to have captured a town near the ruined city of Bakhmut in Ukraine's eastern Donetsk area. Kyiv hasn't confirmed the claim.
Meanwhile, Russia's air attacks have increased in recent weeks. On Tuesday, 18 people were killed and 130 injured in missile assaults on Ukrainian cities.
Ukrainian Defence Minister Rustem Umerov has warned that, although Ukrainian forces face a scarcity of ammunition, Russia has fired over 600 missiles and 1,000 drones in the last two months.
Ukraine, for its part, primarily uses drones to battle. At the weekend, a drone attack triggered an explosion at a major gas export terminal outside St Petersburg.
