The family of a British man captured while fighting with Ukrainian armed forces say using videos of prisoners of war is in breach of the Geneva Convention.
Relatives of Aiden Aslin have spoken of their distress at TV footage of him shown on Russian television - and that he was planning to quit the army before war broke out.
His family also denounced claims that the 28-year-old is a "volunteer, a mercenary, or a spy" as propaganda issued by the Kremlin.
In a statement issued through their MP Robert Jenrick, who represents Newark in Nottinghamshire, the captive's family said: "In 2018 Aiden moved to Ukraine where he met his girlfriend and eventually settled down in Mykolaiv.
"Aiden decided to join the Ukrainian marines and has served in his unit for nearly four years.
"He is not, contrary to the Kremlin's propaganda, a volunteer, a mercenary, or a spy. Aiden was making plans for his future outside the military, but like all Ukrainians, his life was turned upside down by Putin's barbarous invasion.
"He has played his part in defending Ukraine's right to self-determination. The video of Aiden speaking under duress and having clearly suffered physical injuries is deeply distressing.
"Using images and videos of prisoners of war is in contravention of the Geneva Convention and must stop.
"Together with the family of Shaun Pinner (the second Britain captured in Mariupol), we are in contact with the Foreign Office to ensure the Russian authorities meet their obligations to prisoners of war under international law, and ultimately to secure the release of Aiden and Shaun.
"Aiden is a much-loved young man with a strong network of friends around him and a loving wife-to-be. In this difficult time, we urge you to respect the privacy of our family."
Family members previously described Ukraine as Mr Aslin's adopted country.
Images aired on Russian television earlier this week appeared to show Mr Aslin being led around in handcuffs with a cut on his forehead.
The family statement came after Northern Ireland Secretary Brandon Lewis refused to be drawn on suggestions of a possible prisoner exchange when asked about reports of Britons captured while fighting for the Ukrainian military.
In footage broadcast on Russia's Rossiya 24 on Monday, Mr Pinner addressed Boris Johnson and appeared to ask for himself and Mr Aslin to be swapped for pro-Kremlin politician Viktor Medvedchuk, who has been held in Ukraine.
