Paul Urey, a British man captured by pro-Russian forces in Ukraine, has died in detention, Moscow-backed separatists said on Friday.
"He died on July 10," Darya Morozova, a representative of the self-proclaimed Donetsk People's Republic, said on messaging app Telegram, adding that he had diabetes.
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".
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 humanitarian organisation Presidium Network, Urey was a well-travelled humanitarian who worked for eight years in Afghanistan.
Pro-Russian separatists have captured a number of foreign citizens they describe as mercenaries.
Among them are Brits 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.