Victoria Roshchyna, who would have turned 28 this month, disappeared in August last year after travelling to Russian-held east Ukraine on a reporting trip.

She remained missing until April 2024, when her father received a letter from Moscow's defence ministry saying she was being held in Russian detention, according to Ukraine's main journalists' union.

The circumstances of her arrest were not made public and it was not clear where she was being held inside Russia.

Petro Yatsenko, a spokesperson for Ukraine's prison of war coordination headquarters, confirmed on Thursday that she had died in detention.

"In connection with the information about the death of Ukrainian journalist Victoria Roshchyna in Russia, the criminal proceedings opened over her disappearance have been reclassified as a war crime combined with premeditated murder," the Prosecutor General's office said on Friday.

"For many Ukrainian journalists who knew Victoria, her death is a real blow," Ukrainian President Volodymyr Zelensky said on Friday.

"And there are still many other journalists, public figures, community leaders from occupied territory and even ordinary people who were captured during the Russian occupation, in Russian captivity," he added.

The European Union said it was "appalled" by the news and demanded "a thorough and independent investigation that clarifies all the circumstances" of her death, the bloc's foreign affairs spokesperson Peter Stano said.

"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, also in Russia," Stano said in a statement on Friday.

Press rights group Reporters Without Borders said on social media it was "shocked" by Roshchyna's death and offered condolences to her relatives.

Russian news outlet Mediazona reported she died while being transferred to Moscow from a prison in Taganrog, near the Ukrainian border.

Thousands of Ukrainians opposed to Moscow's rule have been detained in occupied territories since Russia's invasion began in 2022, many of whom face torture at the hands of security forces, according to rights groups.

Ukraine said in May more than two dozen Ukrainian media officials were being held in Russian captivity and that negotiations on their return were underway.

Roshchyna worked as a freelancer for various independent news outlets, including Ukrainska Pravda, and had collaborated with the Ukrainian service of US-funded media outlet Radio Free Europe.

In 2022, she was awarded the Courage in Journalism award by the International Women's Media Foundation for her reporting from eastern Ukraine.