The UK foreign secretary, Liz Truss, has raised the case of two Britons sentenced to death for fighting against Russian forces in a phone call with her Ukrainian counterpart. Aiden Aslin, 28, and Shaun Pinner, 48, have been convicted of taking action towards violent seizure of power at a court in the self-proclaimed republic in Donetsk. Truss said she had called the Ukrainian foreign minister, Dmytro Kuleba, on Friday to “discuss efforts to secure the release of prisoners of war held by Russian proxies”. No 10 has said the men are entitled to combatant immunity as prisoners of war. Downing Street has also said that while Boris Johnson was “appalled” at the sentences, there were no plans for direct interventions with Russia, with the emphasis being on their status as members of Ukrainian forces. “The judgment against them is an egregious breach of the Geneva convention,” Truss said. “The UK continues to back Ukraine against Putin’s barbaric invasion.” An adviser to the Ukrainian interior minister said on Friday that Russia had the men sentenced to death in order to gain leverage in its negotiations with Ukraine and its western allies. “The trial of the foreigners raises the stakes in the Russian Federation’s negotiation process. They are using them as hostages to put pressure on the world over the negotiation process,” Vadym Denysenko said. He said Ukraine would coordinate its position on the sentences with Britain, the US and the EU. Ukraine has already sentenced several Russian soldiers to long prison terms for war crimes and Russia may seek to trade the prisoners to get them back. Russia has claimed it had no influence on the proceedings, which took place in a Russian-occupied territory in east Ukraine. “I’d rather not hinder the operation of the judiciary and law enforcement authorities of the Donetsk People’s Republic,” said the Russian foreign minister, Sergei Lavrov, referring to the proxy government. The MPs who represent the two men as constituents, Robert Jenrick, the MP for Newark, and Richard Fuller, the MP for North East Bedfordshire, have called for Russian officials to be summoned to answer for their proxies’ actions in the Ukrainian region. Speaking on BBC Radio 4’s Today programme, Jenrick said: “I’ve urged the foreign secretary to raise this immediately at the highest levels with the Russian government. The UK needs to be clear you can’t treat British nationals in this way. This really is the most egregious breach of international law.” He added: “Aiden and Shaun are not mercenaries, they are combatants, who are prisoners of war now and should be treated in accordance with the Geneva conventions, and the Geneva convention is being breached in the most egregious manner by Russia in holding this kangaroo court and now this sentence to death.” Jenrick said the men were being “hooked out and used in a Soviet-era show trial as a way of taking hostages or taking revenge against the UK”. He said a prisoner exchange could be a solution but that required Russia to “play ball, take this issue seriously and start living up to their international obligations”. Fuller said the men needed access to healthcare and legal advice. He said it was fair to argue they had exposed themselves to risk, but added: “What’s at the centre of this is the recognition by the Russian authorities and their proxies in this region that Shaun and Aiden were members of the Ukrainian military, they are prisoners of war, and that the Geneva convention applies. There appears to be no recognition of that.” On Friday morning the school standards minister, Robin Walker, said the government would use all diplomatic channels to raise the case. He told Sky News: “We utterly condemn the approach that’s been taken here and we will use every method at our disposal to take this issue up.” A Moroccan national, Saaudun Brahim, was convicted alongside Aslin and Pinner. The men were accused of being mercenaries after fighting with Ukrainian troops. The Russian news agency Interfax claimed the men would be able to appeal against their convictions. The Russian foreign ministry spokesperson Maria Zakharova has described the British reaction to the sentences against the men as “hysterical” and said a UK appeal should be directed at the self-proclaimed Donetsk republic, a Russian-occupied territory internationally recognised as part of Ukraine. Aslin is originally from Newark-on-Trent in Nottinghamshire, and Pinner is from Watford, but his mother lives in Fuller’s constituency. They were both members of regular Ukrainian military units fighting in Mariupol, the southern port city that has been the scene of some of the heaviest fighting since Vladimir Putin’s invasion of Ukraine. Concerns were also raised in Ukraine about the status of Andrew Hill, 35, who was captured in fighting in southern Ukraine. Unlike the other two Britons, Hill is a member of the International Legion, the group of several thousand volunteer soldiers who have agreed to fight as part of Ukraine’s army during the war. A spokesperson for the Legion said they were worried about Hill’s welfare, who local reports had suggested was also going to be put on trial alongside Aslin and Pinner. “Then the trial came and went and it turned out that Andrew Hill was not among those sentences, which raises the question of what has happened to him. What’s his status? Is he even alive?”.