The Russian defence ministry said six Russian crew members and three Russian soldiers had been on the Ilyushin Il-76 military transport plane shot down near the Russian city of Belgorod near the Ukrainian border.
"The Ukrainian leadership was well aware that, in accordance with established practice, Ukrainian servicemen would be transported by military transport aircraft to the Belgorod airfield today to be exchanged," a ministry statement said.
"According to an earlier agreement, this event was to take place in the afternoon at the Kolotilovka checkpoint on the Russian-Ukrainian border," it said.
"By committing this terrorist act, the Ukrainian leadership has showed its true face. It disregarded the lives of its own citizens."
Russia's foreign ministry called the shooting down "a barbaric act".
Mykhailo Podolyak, a Ukrainian presidential adviser, told Reuters: "Comments will come a little later. Time is needed to clarify all the data."
Video posted on the Telegram messaging app by Baza, a channel linked to Russian security services, and verified by Reuters, showed a large aircraft falling towards the ground near the village of Yablonovo in the Belgorod region and exploding in a vast fireball.
Ukraine's GUR military intelligence spokesperson Andriy Yusov told the Radio Svoboda outlet that a prisoner exchange had been planned for Wednesday, adding: "It's not taking place at the moment."
Ukraine's defence ministry did not immediately reply to a Reuters request for comment. It was cited by the Interfax-Ukraine news agency as saying it did not have any reliable information for now but would share it when it did.
Ukrainian media outlet Ukrainska Pravda initially cited military sources as saying Kyiv had shot down the plane because it was carrying S-300 missiles, but later corrected the story saying that information had not been confirmed by other sources.
Andrei Kartapolov, a lawmaker in Russia's parliament and a retired general, said in a TV interview with the SHOT outlet that it was impossible for the operators of Ukrainian surface-to-air missile systems to mistake transport planes for military planes or helicopters as targets.
"It was done deliberately to sabotage the prisoner exchange," said Kartapolov, saying a second Russian Il-76 transport plane carrying around 80 Ukrainian soldiers to the exchange had managed to turn around.
The Russian defence ministry said its radar operators had detected the launch of two Ukrainian missiles at the time the plane was downed.
Kartapolov, who has close links to the Russian defence ministry, said the plane had been downed by three missiles of either U.S. or German manufacture. He said it had not been escorted by Russian fighter planes because the flight had been agreed with the Ukrainians.
If the details are confirmed, it would be the deadliest incident of its kind inside Russia's internationally recognised borders during the almost two-year-old war.
Reuters could not immediately verify details of who was on board the downed plane, but Moscow and Kyiv have regularly swapped prisoners since Russia began what it calls its "special military operation" in Ukraine in February 2022.
Russian state media published a list of names of the 65 captured Ukrainian soldiers it said were on board along with their dates of birth.
GOVERNOR SAYS NO SURVIVORS
Vyacheslav Gladkov, governor of Belgorod region, said all 74 people on board the plane had been killed and that the plane had come down in the region's Korochansky district, northeast of the city of Belgorod.
He said investigators and emergency workers were already on the scene.
The Belgorod region, which borders Ukraine, has come under frequent attack from Ukraine in recent months, including a December missile strike which killed 25 people.
Vyacheslav Volodin, chairman of the State Duma, the lower house of Russian parliament, said the chamber would prepare an appeal to the U.S. Congress and the German parliament "so their lawmakers at last can see clearly who they are financing, who they are helping."
"They (Ukraine) shot down their own soldiers in mid-air. Their mothers, wives and children were waiting for them. They took a decision and shot down our defenceless pilots on a military transport plane, who were carrying out a humanitarian mission, with American and German rockets," said Volodin.
The Il-76 is a military transport aircraft designed to airlift troops, cargo, military equipment and weapons. It usually has a crew of five, and can carry up to 90 passengers.
The Kremlin said in response to a reporter's question that it was looking into the situation.
