The Russian army and Moscow-backed separatists on Friday accused Kyiv's forces of striking a jail holding Ukrainian prisoners of war in eastern Ukraine, saying dozens of people died and scores were wounded.
The Russian defence ministry said the jail in Olenivka in the separatist-held region of Donetsk had been targeted overnight with US-made HIMARS rocket systems.
"Forty Ukrainian prisoners of war were killed and 75 wounded," the defence ministry said in a statement, adding that eight employees of the detention centre were also injured.
Denis Pushilin, leader of the Donetsk separatists, put the death toll at 47 people. Territorial defence forces of the separatist statelet said that 53 people died.
A total of 193 people were held in the jail at the time of the strike, Pushilin said in comments broadcast on Russian state television.
The Russian defence ministry said the Ukrainian prisoners of war included members of the Azov battalion, who defended the Azovstal plant in Ukraine's port city of Mariupol.
Russia describes the former paramilitary unit, which has previous links to far-right groups, as a neo-Nazi organisation.
Russian television showed what appeared to be destroyed barracks and tangled metal beds but no casualties could be seen.
Moscow claimed that the "bloody provocation of the Kyiv regime" was designed to discourage Ukrainian troops from laying down their arms and surrendering.
"This egregious provocation was carried out to intimidate Ukrainian servicemen," the defence ministry said.
Pushilin, the Donetsk leader, claimed Kyiv forces struck the jail because Ukrainian prisoners of war had begun to testify.
After a weeks-long siege and resistance at the Azovstal steel plant, the last stronghold of Mariupol, around 2,500 Ukrainian fighters surrendered in May.