Relatives of Aiden Aslin also denounced claims that the 28-year-old was a “volunteer, a mercenary, or a spy”, as claimed in propaganda issued by the Kremlin. Mr Aslin is in Russian custody after being captured in the southern besieged port of Mariupol, alongside fellow British fighter Shaun Pinner, 48.
Prime Minister Boris Johnson urged Russia to treat the captives with compassion.
And Mr Aslin’s family, in a statement issued through their MP Robert Jenrick, who represents Newark in Nottinghamshire, 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.”
Of Mr Aslin, Mr Johnson told the House of Commons he had served in the Ukrainian army for some time and was not a mercenary.
And he urged the Russian state to treat him “humanely and compassionately”.
“Although… we actively dissuade people from going to that theatre of conflict, I understand that he’d been serving in the
Ukrainian forces for some time and his situation was very different from that of a mercenary,” Mr Johnson said.
“I hope that he is treated with care and compassion.”
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.
He also warned Britons not to travel to Ukraine to fight.
In footage broadcast on Russia’s Rossiya 24 earlier this week, 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.
Mr Pinner is a former Royal Anglian soldier and has made clear he considers Ukraine his second country, where he relocated in 2018 and married a local.
His family has also expressed their distress at the situation.
An earlier statement issued by his family said: “Shaun was a well-respected soldier within the British Army serving in the Royal Anglian Regiment for many years. He served in many tours including Northern Ireland and with the United Nations in Bosnia.
“In 2018 Shaun decided to relocate to Ukraine to use his previous experience and training within the Ukraine military.
“Shaun enjoyed the Ukrainian way of life and considered Ukraine as his adopted country over the last four years. During this time, he met his Ukrainian wife, who is very focused on the humanitarian needs of the country.
“He progressed into the Ukrainian Marines as a proud member of his unit.”
“We would like to make it clear he is not a volunteer nor a mercenary, but officially serving with the Ukrainian Army in accordance with Ukrainian legislation.
“Our family is currently working with the Foreign Office along with the family of Aiden Aslin, who is also being held by the Russian Army to ensure their rights as prisoners of war are upheld according to the Geneva Convention.”