Viktoria Roshchyna, 27, disappeared in August 2023 after embarking on a reporting trip to occupied eastern Ukraine, and Russia acknowledged last April that she was being held.

Ukrainian human rights ombudsman Dmytro Lubinets confirmed her death on social media late on Thursday in what he condemned as illegal detention. He did not specify the circumstances.

Andriy Yusov, a spokesperson for Ukraine's military intelligence, told the Ukrainian public broadcaster Suspilne that Roshchyna had been on a list of prisoners to be exchanged, and that "everything necessary had been done" for the swap.

She had been due to be transferred to Moscow from the southern city of Taganrog, he said.

The campaign group Reporters Without Borders said Roshchyna had died on September 19, citing a letter that her family received on Thursday from Russia's Defence Ministry.

The Ukrainian Prosecutor General's office said it had updated its war-crime investigation into Roshchyna's disappearance to include murder.

On Friday, the Ukrainian delegation to the European Union demanded "a thorough and independent investigation that clarifies all the circumstances" of Roshchyna's death.

"Her fate is a tragic reminder of the many thousands of persons detained in occupied Ukrainian territories and Russia, as well as the repression imposed by Russian authorities," it said.