The overnight attack in Zaporizhzhia also damaged surrounding homes, the regional leader, Ivan Fedorov, said.
Russian forces launched eight strikes using high-explosive aerial glide bombs, he added.
The Ukrainian president’s Chief of Staff, Andriy Yermak, condemned the strikes as “another war crime” committed by Russia.
Zaporizhzhia is one of four eastern regions in Ukraine that Russia claims to have annexed since 2022, although the region is largely under Ukrainian control.
Ukraine’s justice ministry said four bombs hit the Bilenke penitentiary, destroying the dining hall, administrative headquarters, and quarantine area, and that all those killed and wounded were inmates.
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Russian forces have frequently targeted Zaporizhzhia since the start of the full-scale invasion of Ukraine in 2022.
Ukraine’s human rights commissioner said attacking a prison was a gross violation of humanitarian law as people in detention did not lose their right to life and protection.
US President Donald Trump issued a stark ultimatum to Moscow Monday, warning that Russia had “about 10 or 12 days” to agree to a ceasefire or face sweeping sanctions.
Speaking during a visit to Scotland, Trump told reporters he would “announce it probably tonight or tomorrow,” adding, “there’s no reason to wait, if you know what the answer is”.
Earlier in July, Trump set a 50-day deadline for the Kremlin to reach a truce with Kyiv or risk economic penalties, but the warning has not halted Russia’s barrage of strikes.
There were further casualties in a missile and drone attack late on Monday in the Dnipropetrovsk region of eastern Ukraine.
A strike on the industrial city of Kamyanske left two people dead and five injured, according to regional head Serhiy Lysak.
Another person was killed and several were wounded in the Synelnykivsky district, while a 75-year-old woman was killed and a 68-year-old man injured when their home was hit in a village late on Monday.