STORY: Briton John Harding, Croatian Vjekoslav Prebeg and Swedish citizen Mathias Gustafsson, who were captured in and around the port city of Mariupol, face a possible death sentence under the laws of the self-proclaimed Donetsk People's Republic, Russian state-owned news agency TASS reported.
Two more Britons, Dylan Healy and Andrew Hill, were also charged but do not face execution. All five of the accused pleaded not guilty to the charges, TASS reported.
It cited the judge as saying that the trial would resume in early October.
In response to the charges against Prebeg, the Croatian Foreign Ministry said: "Croatia dismisses the indictment and does not consider it to be founded and legal because it is opposed to international law and international conventions on the treatment of detained civilians and prisoners of war."
The British Foreign Office did not immediately respond to a request for comment.
The Donetsk authorities in June sentenced to death two Britons and one Moroccan citizen captured fighting with Ukrainian forces against Russia on charges of attempting to forcibly seize power, and of being mercenaries.
Foreign governments have declined to negotiate with the Donetsk People's Republic, one of two Russian-backed entities that have controlled parts of east Ukraine's Donbas region since 2014, citing its internationally recognized status as part of Ukraine.