Kiev is pushing for a prisoner swap with Moscow to bring home Ukrainians whom Russia is accused of jailing on political grounds, including several who are now on hunger strike.
"We say again and again to the Russian Federation: take back yours and release the Ukrainians," said Iryna Herashchenko, the deputy speaker of Ukraine's parliament, after revealing the names of 23 Russian prisoners whom Kiev is willing to free.
"These people are convicted of or detained for crimes against the territorial integrity and sovereignty of Ukraine, for preparing terrorist attacks, spying and other serious crimes," she added.
“Ukraine is willing to pardon them, if Russia reciprocally releases the Kremlin’s political prisoners.”
After Ukraine ousted Kremlin-backed leaders in a 2014 revolution, Russia annexed Crimea and fomented a separatist conflict in eastern Ukraine that has claimed more than 10,300 lives.
The former allies accuse each other of using illegal arrests and wrongful convictions to suppress political dissent, and of using detainees as bargaining chips for prisoner exchanges that are often agreed and conducted in secrecy.
Ms Herashchenko said the Ukrainians who should be freed include Oleh Sentsov, a Crimean film director jailed for 20 years on spurious terrorism charges; Volodymyr Balukh, an activist convicted of weapons possession after flying a Ukrainian flag over his house in occupied Crimea; and Pavlo Hryb, who was abducted in Belarus and resurfaced in a Russian jail facing terrorism allegations.
Mr Sentsov has been on hunger strike for 51 days and Mr Balukh for 107 days; at least two other Ukrainian prisoners in Russia, Stanislav Klykh and Oleksandr Shumkov, have reportedly been on hunger strike for a shorter time.
Last month, the European Parliament passed a resolution urging Moscow to free "more than 70 Ukrainian political prisoners in Russia and in the occupied Crimea".
Ambassadors in Kiev from the G7 powers, meanwhile, said they were "deeply concerned" about Mr Sentsov and other Ukrainians jailed in Russia, and called for their release "as part of a broader bilateral exchange of detainees".
Rights advocates hope the Kremlin may free the prisoners to burnish its reputation during the football World Cup, and ahead of a planned summit between Russian president Vladimir Putin and US counterpart Donald Trump in Helsinki on July 16th.
"The clock is ticking," said Tanya Lokshina, Russia programme director for Human Rights Watch.
“There’s still time for [world football’s governing body] Fifa and key international actors to use their leverage and urge Russia to free these unjustly detained people before the tournament’s last game.”
Last week, however, Russia prevented Ukraine's human rights commissioner, Lyudmyla Denisova, from visiting Mr Sentsov in a remote jail known as the "Polar Bear" at Labytnangi, inside the Arctic Circle.
Moscow denies holding political prisoners and accuses Kiev of wrongly arresting Russian citizens.
In May, Ukraine levelled treason charges against Kirill Vyshinsky, a journalist working for a Russian state news agency in Kiev; less than a month later, a Moscow court sentenced Ukrainian reporter Roman Sushchenko to 12 years in jail for spying.