LONDON (AP) — A British aid worker detained by pro-Russia separatists in Ukraine has died in captivity, a separatist official and a U.K. charity involved with his case said Friday.

The British government summoned the Russian ambassador in London to demand an explanation.

Daria Morozova, the human rights ombudswoman for the Moscow-backed separatist leadership in the Donetsk region of eastern Ukraine, said a British “mercenary,” whom she named as Paul Urey, died in captivity on Sunday. She said he had died of chronic illnesses and stress.

The Presidium Network aid charity said Urey’s family had been notified of his death by British officials.

Britain's Foreign Office said it had summoned Russian Ambassador Andrey Kelin “to express the U.K.’s deep concern.”

Foreign Secretary Liz Truss said she was “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,” she said.

Urey, 45, was detained in April at a checkpoint near Zaporizhzhia, some 470 kilometers (290 miles) southeast of Kyiv, along with another British man, Dylan Healy. The two men had been operating on their own in the war zone, helping to evacuate civilians.

Presidium Network co-founder Dominik Byrne said Urey had diabetes and needed a regular supply of insulin.

“It’s obvious that his welfare was not looked after,” Byrne said. “The Russian authorities and the Donetsk People’s Republic knew he had need of insulin but all the way through this the Red Cross has been denied welfare access to him and has never been able to verify his actual conditions in prison.”

Byrne said “multiple agencies” including the British government and the Red Cross had tried in vain to secure Urey’s release.

“We are formally calling for his captors to release his body and help us repatriate it back to the U.K. for his family,” he said. “We really feel that is of ultimate importance and the least they can do at this stage.”

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