The body of a British aid worker who died in Russian detention shows signs of "possible unspeakable torture", Ukraine's Foreign Minister Dmytro Kuleba has said.
Russian authorities claimed the aid worker, from Manchester, had died as a result of “illness and stress”.
His body was returned on Wednesday.
“Russians have returned the body of a British humanitarian worker Paul Urey whom they captured in April and reported dead due to ‘illnesses’ and ‘stress’ in July,” Mr Kuleba tweeted.
“With signs of unspeakable torture, detaining and torturing civilians is barbarism and a heinous war crime.”
Mr Kuleba said Urey, a Type 1 diabetic, was a “brave man who dedicated himself to saving people".
“Ukraine will never forget him and his deeds,” he added.
"We will identify perpetrators of this crime and hold them to account. They won't escape justice."
Russian ambassador to the UK, Andrei Kelin, was summoned to the Foreign Office to face questioning over what happened to Urey, who was detained near the Ukrainian city of Zaporizhzhia in April.
British Prime Minister Liz Truss, who was foreign secretary at the time, said Russia “must bear the full responsibility” over his death.
Urey’s mother Linda said she was "truly angry" in a post on Facebook.
She said her son had been taken from her at birth and, after finding her, he had been taken from her again.
"Cruel cruel world," she added.
Ms Urey told Sky News at the time of his capture that she had begged her son not to go to Ukraine.
She added: "He said, 'Mama I can't live with myself knowing people... need help to get to a safe place, I have to go. I would feel bad'."
He was captured alongside fellow Briton Dylan Healey at a checkpoint south of the city of Zaporizhzhia in south-east Ukraine while driving to help a woman and two children.
Four Britons have now died in Ukraine during the conflict. They include Craig Mackintosh, from Thetford, Norfolk, who was killed on August 24. He was volunteering as a medic at the time of his death.