Three men from Britain and two from Sweden and Croatia, who were captured in eastern Ukraine, went on trial in a court administered by Kremlin-backed separatists in Donetsk, Russian media reported.
John Harding, Croatian Vjekoslav Prebeg and Swedish citizen Mathias Gustafsson face a potential death sentence under the laws of the self-proclaimed Donetsk People's Republic, Russian state-owned news agency Tass said on Monday.
Dylan Healy and Andrew Hill have also been charged but will not be sentenced to execution. All five pleaded not guilty to the charges, Tass reported.
Mr Harding, Mr Prebeg and Mr Gustafsson were captured in the Ukrainian port of Mariupol and are accused of attempting to “seize power by force” and “taking part in armed conflict as mercenaries”, the RIA Novosti news agency reported.
Mr Hill faces charges of being a mercenary, while Mr Healy is being tried for taking part in the recruitment of mercenaries for Ukraine, the news agency said.
Their next hearing is scheduled for October, Russian media reported. The British Foreign Office did not immediately respond to a request for comment.
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On June 9, the supreme court of the self-proclaimed republic sentenced two Britons and a Moroccan to death for being mercenaries. The three men, captured by pro-Russian forces in Ukraine's industrial east, have appealed.
There has been a moratorium on the death penalty in Russia since 1997, which does not apply in the two separatist regions in Ukraine.
Another British volunteer, Paul Urey, 45, died in captivity last month, Moscow-backed separatists in eastern Ukraine said.
UK Foreign Secretary Liz Truss said Russia “must bear the full responsibility” over his death.
In April, the Presidium Network, a non-profit group, said Urey and Mr Healy had been captured at a checkpoint in south-east Ukraine.
Urey, who was born in 1977 and was from Manchester, and Mr Healy, born in 2000 and from Cambridgeshire, travelled to Ukraine of their own accord, the organisation said.
They were not working for the Presidium Network, which helps to provide aid to Kyiv.
The group said the pair went missing while driving to help a woman and two children.