Russia says its soldiers recently released as part of a prisoner swap with Ukraine were "beaten" and "tortured with electricity in captivity."
The Russian Investigative Committee, which probes major crimes, said in a statement on Tuesday that it was "verifying facts of inhuman treatment of Russia soldier prisoners in Ukraine."
Moscow and Kiev exchanged 144 prisoners of war each last week.
The investigative committee said the freed Russian soldiers had told investigators about "the violence they had suffered."
One of the soldiers, according to the Russian statement, said Ukrainian medics had treated him without applying anesthetics and that he was "beaten, tortured with electricity in captivity." He said he was left without food and water for days.
Another Russian soldier said he was badly beaten and had his wound irritated by Ukrainian medics.
The Russian committee said the testimonies of the freed soldiers were examples of "violations of the Geneva Convention on prisoners of war."
Russian President Vladimir Putin declared a military campaign against Ukraine on February 24.