Russia bears "full responsibility" over the reported death of Paul Urey, British Foreign Secretary Liz Truss said Friday, after Moscow-backed separatists in Ukraine said the captured Briton had died in detention.
"I am shocked to hear reports of the death of British aid worker Paul Urey while in the custody of a Russian proxy in Ukraine," she said. "Russia must bear the full responsibility for this."
Darya Morozova, a representative of the self-proclaimed Donetsk People's Republic, said on the messaging app Telegram that Urey "died on July 10" and that he had diabetes.
Truss said that Urey "was in Ukraine to try and help the Ukrainian people in the face of the unprovoked Russian invasion.
"The Russian government and its proxies are continuing to commit atrocities.
"Those responsible will be held accountable. My thoughts are with Mr Urey's family and friends at this horrendous time," added the minister, who is in the running to become the UK's next leader.
Non-governmental organisations describe Urey as a humanitarian who worked as an aid volunteer in Ukraine.
Moscow-backed separatists insist Urey was a "professional" soldier and took part in conflicts in Afghanistan, Iraq, Libya and Ukraine.
Morozova, the breakaway region's ombudsperson, said that the Briton took part in fighting in Ukraine and also recruited and trained mercenaries before his capture in April.
Urey, born in 1977, was a Type 1 diabetic and needed regular insulin doses, according to his mother Linda Urey, who had earlier said his family was "extremely worried".
"He was out there on his own accord. We want everyone's support to bring (my) son home and pray he is safe," she said after he was detained.
Morozova said he suffered from a number of chronic diseases and was also "in a depressed psychological state".
"Despite the severity of the alleged crime, Paul Urey was given appropriate medical assistance," she said.
"However, taking into account his diagnoses and stress, he died on July 10."
Morozova also accused the International Committee of the Red Cross of refusing to provide Urey with necessary medicine.
According to the humanitarian organisation Presidium Network, Urey was a well-travelled humanitarian who worked for eight years in Afghanistan.
Pro-Russian separatists have captured several foreign citizens they describe as mercenaries.
Among them are Britons Aiden Aslin and Shaun Pinner who in June were sentenced to death by separatist authorities in the stronghold of Donetsk.
Britain has expressed fury over the death sentences handed to the two Britons in the case.
Urey and Dylan Healy, another aid worker, were "captured" by Russian soldiers in April at a checkpoint south of the city of Zaporizhzhia in south-eastern Ukraine, according to Presidium.
The pair were said to be operating independently to try to get vulnerable Ukrainians out and were arrested as they drove to help a woman and her two children in Zaporizhzhia.