The United States, France, Britain and 35 other countries asked UN Secretary-General Antonio Guterres on Thursday to raise the case of jailed film director Oleg Sentsov and dozens of other Ukrainian Prisoners during his talks with Russian President Vladimir Putin next week.
The UN chief will hold talks with Putin on Wednesday and attend a Portugal-Morocco match during the World Cup that Russia is hosting.
The 38 countries wrote in a letter to Guterres that the case of Sentsov, who has been on a hunger strike since May 14, should be raised as a "matter of urgency" and appealed to him to try to mediate a solution.
"The UN's engagement on these concerns is welcome and we would encourage further steps to address the plight of all those unlawfully detained," said the letter seen by AFP.
Among the signatories were Australia, Canada, many EU countries along with Turkey, Moldova, Georgia and Ukraine.
Ukrainian Ambassador Volodymyr Yelchenko on Thursday presented Guterres with the letter and a list of dozens of detainees including trade unionist Oleksandr Kolchenko, historian Stanislav Klykh and politician Mykola Karpyuk.
The most high-profile case is that of Sentsov, a 41-year-old filmaker who is a serving a 20-year sentence in a Russian jail on charges of plotting terrorist attacks. He denies the charges.
Putin has recently hinted that talks were taking place with Ukraine on a prisoner swap that could take place during the World Cup.
More than 10,000 people have been killed in the war in east Ukraine between government forces and pro-Russian separatists.
A political deal reached in 2015 and backed by France and Germany has failed to yield progress toward ending the conflict.