Britain's foreign office on Friday summoned Russian Ambassador Andrei Kelin to express "deep concern" over reports of the death of a British aid worker.
"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. Russia must bear the full responsibility for this," Foreign Secretary Liz Truss said.
"Those responsible will be held to account."
An official in the self-proclaimed Donetsk People's Republic (DPR) announced Paul Urey's death earlier in the day.
His death was also confirmed by British charity Presidium Network, which described him as a humanitarian worker and denied he had any military background.
Urey, 45, was detained and charged with "mercenary activities" by separatists in the DPR, a breakaway entity that is recognised only by Russia, Syria and North Korea.
(Reporting by William James and Muvija M)
