Ukraine's human rights commissioner has accused Russia of refusing to exchange prisoners as "Moscow doesn't want them back".
Kyiv and Moscow have held multiple prisoner swaps since the war broke in February 2022.
Ukraine has since said that a prisoner exchange has not taken place since early August.
"Exchanges don't happen because Russia doesn't want them to," said Dmytro Lubinets on Telegram.
"No one from the Russian side wants to take them back."
Mr Lubinets went to claim that Russian prisoners held in Ukraine had requested to be exchanged in order to return home.
Ukraine has already brought back 2,598 people from Russian captivity during 48 swaps, the Strategic Communications Department of the Armed Forces said on Tuesday.
Mr Lubinets added: "All the initiatives, desires and actions of Ukraine regarding the return of our defenders from captivity are met by a Russian unwillingness to return its citizens."
In November, the Ukrainian government said it had registered 3,574 Ukrainian military and 763 civilians taken into Russian or Moscow-backed separatists' captivity since 2014.
The figure included those who have already returned to Ukraine, it said. However, it said the numbers did not show all the current prisoners.
The Russian Defence Ministry has not commented on the claim and the numbers have not been independently verified.
A Ukrainian military spokesperson added on Wednesday that Ukrainian troops were trying to push Russian forces back from the eastern bank of the river, which serves as a formidable natural barrier on the battlefield.
Vladimir Saldo, the Russian-installed governor of the part of Kherson region which Moscow controls, acknowledged in a statement that Ukrainian forces had managed to cross the river, but said they were taking heavy losses.
