Relatives of a British man who was taken prisoner while serving with the Ukrainian marines have spoken of their distress at seeing him paraded in front of cameras by his Russian captors.

Aiden Aslin’s family said he had been a legitimate combatant with the Ukrainian armed forces, and was speaking under duress during an interview posted online this week that was described as a breach of the Geneva conventions.

“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,” his relatives said.

Images of Aslin in handcuffs appeared last week shortly after he had indicated via his social media accounts, which are controlled by a third party, that he had no choice but to surrender because the unit he was serving with inside the besieged city of Mariupol had run out of food and ammunition.

This week, a 45-minute film was released online, during which he was questioned by Graham William Phillips, a British man who has previously worked for the Kremlin-backed television channel RT and been accused of spreading Moscow’s propaganda.

During the film, Aslin answers no when asked several times if he is speaking under duress. Questioned by Phillips, he recounts his time with the Ukrainian military and, at his interviewer’s prompting, repeats several of Moscow’s propaganda lines, including that he is a mercenary and not, therefore, a legitimate combatant.

“The video of Aiden speaking under duress and having clearly suffered physical injuries is deeply distressing,” a statement from his family released on Tuesday said. “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, 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 Sean,” they said, referring to a second British soldier captured in Ukraine.

The statement added: “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.”

Boris Johnson’s official spokesperson said on Tuesday the government would “provide whatever support we can”, when asked about the fates of British nationals held by Moscow’s forces. “We would call on Russia and Putin to treat any prisoners of war properly, in line with all war conventions and laws.”

Asked about attempts to use the captured men for propaganda purposes, the spokesperson said: “We’ve seen Putin’s regime use captured prisoners and soldiers in this way, but we must not be swayed by any attempt to spread disinformation or to distract from who is responsible.”

Aslin’s local MP, Robert Jenrick, added: “The misuse of Aiden Aslin by the Russian government for propaganda purposes, including featuring him in videos clearly filmed under duress, is a disgraceful and flagrant breach of the Geneva convention. Russia must desist from this illegal behaviour immediately and treat Aiden appropriately.”

He also called on online platforms to live up to their “responsibility to take down prisoner of war-related content from their sites”. He said: “I urge YouTube to remove the grotesque video of Aiden Aslin immediately. Aiden’s family and I are working with the Foreign Office to ensure his swift and safe release.”