Two British fighters Shaun Pinner, 48, and Aiden Aslin, 28, captured by Moscow's forces in Ukraine, have appeared on Russian state TV and asked to be exchanged for pro-Russian politician Viktor Medvedchuk.
The families of Shaun Pinner and Aiden Aslin have said the men are not mercenaries or volunteer fighters and should therefore be treated as prisoners of war under the Geneva Convention.
The two men asked British Prime Minister Boris Johnson to help bring them home in exchange for Ukraine releasing captured oligarch Mr Medvedchuk, who was shown in a video released around the same time on Monday by Ukraine's SBU intelligence service asking to be swapped too.
The Foreign Office called on the Kremlin to treat prisoners humanely.
Both Britons had been living in Ukraine before it was invaded and had Ukrainian partners. They had both been serving as marines in Ukraine's military.
In an earlier social media video Mr Pinner, who is originally from Bedfordshire, gives his name and says he was fighting in the besieged port city of Mariupol before being captured.
He says he is now in the breakaway Donetsk People's Republic area in eastern Ukraine. Again, it is not clear if he was speaking under duress.