The body of a British aid worker captured by Russian proxies showed signs of torture, a Ukrainian official says.

A British aid worker who died in custody after being captured by pro-Russian forces in Ukraine appears to have suffered “possible unspeakable torture,” according to the Ukrainian government.

The 45-year-old aid worker, Paul Urey, was captured in April with another Briton, Dylan Healy, by Russian forces at a checkpoint near Zaporizhzhia in southeastern Ukraine while working to evacuate civilians there.

Mr. Urey died in July, according to a pro-Russian separatist group and a humanitarian organization that was in contact with him while he was detained. Russian officials had previously claimed that Mr. Urey died in detention from “illness” and “stress.”

“Detaining and torturing civilians is barbarism and a heinous war crime,” Ukraine’s foreign minister, Dmytro Kuleba, said in a Twitter post on Wednesday, adding that Mr. Urey’s corpse bore signs of possible torture after it had been returned by Russian officials.

“We are disturbed by reports that aid worker Paul Urey may have been tortured in detention,” a spokesperson for Britain’s Foreign Office told Sky News on Thursday. “It is essential that we see the results of a full post-mortem as soon as possible.”

Mr. Urey was reportedly charged in April with “mercenary activities” in the so-called Donetsk People’s Republic, the Moscow-backed breakaway enclave in occupied eastern Ukraine.

“From our side, he was given the necessary medical assistance,” the human rights ombudsperson for the Kremlin, Daria Morozova, said in a Telegram post after his death in July. She attributed his death to health issues including diabetes, respiratory damage and possible depression.

Britain’s Foreign Office summoned the Russian ambassador in July to respond to Mr. Urey’s death. Liz Truss, who was then serving as Britain’s foreign secretary, and took over as prime minister this week, said in July that she was “shocked” to learn of his death “while in the custody of a Russian proxy in Ukraine.”

Mr. Urey’s mother, Linda Urey, wrote in a Facebook post on Thursday that she was trying to bring his body back to England from Ukraine. “I’m just hoping he can come home now,” she said.

She asked people to contribute to a GoFundMe campaign organized by his 20-year-old daughter, Chelsea Coman, to repatriate his body. His family hoped to give him a burial in his home country “with people who love him and celebrate his life,” his daughter wrote on the fund-raiser’s webpage.