He said: “I am Shaun Pinner. I am a citizen of the United Kingdom. I was captured in Mariupol.
“I am part of 36th brigade, 1st Battalion Ukrainian Marine.
“I was fighting in Mariupol for five to six weeks and now I am in the Donetsk People’s Republic.”
Mr Pinner, who previously served in the Royal Anglian Regimen, was fighting as a “contract soldier” alongside Ukrainian forces.
He was shown on a military TV channel linked to war journalist Andrey Rudenko.
He reportedly told the Russian military: “He doesn’t want war and wants to go home.”
It was announced on Saturday that Mr Pinner is set to be interrogated by the Russian Investigative Committee.
In the TV broadcast, he was described as “an English mercenary who served in the 36th brigade of the Ukrainian Armed Forces Marines as a stormtrooper and is now in captivity.
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Mr Aslin, 28, joined the Ukrainian marines four years ago.
He has dual UK-Ukrainian citizenship and a Ukrainian fiancee.
However, Russian forces have reportedly dubbed him an enemy rather than a prisoner of war.
This is an important distinction as, under Russian law, those suspected of espionage face interrogation and lengthy prison sentences.
While PoWs are released at the end of hostilities, convicted spies may remain behind bars, possibly until an exchange of agents can be arranged.
TV reporter Andrey Rudenko said in a Russian news broadcast that Mr Aslin was an English mercenary “used [by the West] to perform various delicate tasks”, adding: “I am sure he will be able to shed light on the actions of Western intelligence agencies in Ukraine.”
Mr Aslin, who is from Newark, Nottinghamshire, was shown on TV with his face bruised.
It is believed he is being held in a military detention facility.
Footage also showed him being questioned by one of his captors off camera and he appears to agree with the suggestion that “those who stood with you [in Ukraine], they are killers”.