The Foreign Office has condemned the exploitation of prisoners of war after two British men held by Kremlin forces in eastern Ukraine were charged with "mercenary activities".
Dylan Healy, 22, and Andrew Hill, 35, were both reported to have been captured in April.
Mr Healy, from Cambridgeshire, was in Ukraine as an aid worker for British non-profit organisation Presidium Network when he was seized at a checkpoint south of the city of Zaporizhzhia.
He was taken alongside another British national, Paul Urey, 45, who has been described as a family man with children.
Mr Urey was not mentioned in the announcement about the charges against Mr Healy and Mr Hill and no known charges have been made.
Mr Hill, a military volunteer, was filmed with a bandaged left arm and a makeshift dressing on his head in footage aired on Russian TV in April.
Now, TASS news agency has reported that officials in Moscow-backed Donetsk People's Republic (DPR) have been charged - and say both Mr Healy and Mr Hill have refused to cooperate.
A DPR source told TASS: "Criminal cases have been initiated and charges were presented for (mercenarism) against British citizens Dylan Healy and Andrew Hill, currently in detention in DPR.
"Investigation operations are under way as the investigators look for evidence of the crimes, committed by the British, because they do not want to testify and refuse to cooperate on their criminal cases."
However, Presidium Network, which has been assisting Mr Healy's family since April, has said the charges are "politically motivated" and a "fabrication" by the DPR.
Mr Healy's friend, Allan Moore, said he had spoken to the aid worker's mother who is "taking guidance" about who to speak to.
Mr Moore told Sky News: "It's just madness, to be honest. He is definitely not a mercenary. He was just volunteering."
The Red Cross and the British government have been in touch with Mr Healy and those who have been holding him, Sky News understands.
The Foreign Office said in a statement: "We condemn the exploitation of prisoners of war and civilians for political purposes and have raised this with Russia.
"We are in constant contact with the government of Ukraine on their cases and are fully supportive of Ukraine in its efforts to get them released."
Presidium Network said it "has been made aware of the unverified mercenary charges levied against Dylan Healy through the Russian State Media."
The organisation said it provided evidence to demonstrate Mr Healy was an "independent humanitarian volunteer" in Ukraine at the time of his capture and not attached to any military or paramilitary unit.
"At no time did he participate in any military action," a statement said.
"The charges brought by the DPR are not supported by evidence and can therefore only be explained as a politically motivated action by DPR and the Russian government.
"We condemn the fabrication of charges by the DPR for political gain and misrepresentation of Dylan's role in the Ukraine."
According to a pro-Kremlin website, Mr Healy and Mr Hill will face the same mercenary charges as British military volunteers Aiden Aslin and Shaun Pinner.
Mr Aslin, 28, from Newark in Nottinghamshire, and Mr Pinner, 48, from Bedfordshire, were sentenced to death at a DPR court in June.
But on Thursday, the European Court of Human Rights (ECtHR) intervened in their cases, warning Moscow that it should ensure the death penalty is not carried out.
Stanislav Aseyev, a Ukrainian, was held by the same group for two years until he was released in a prisoner exchange in 2019.
Speaking to Sky News of his own experiences he claimed to have been tortured with electricity.
"I hope that because of the publicity of this case, they will be treated much better than me and the other prisoners," he said.
He cautioned that "there is no legal system at all in that territory" and warned the British government to take the threat of the death sentence seriously.
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