Amnesty International says both warring sides in eastern Ukraine are almost daily perpetrating war crimes, including torture and summary killings of prisoners.
In a new report released on May 22, the rights group said it has heard accounts from former captives of government and separatist forces of savage beatings, torture with electric shocks, kicking and stabbings.
Amnesty says it interviewed 17 captives of the separatists and 16 others held by Ukrainian government forces for its report.
Concern about the treatment of prisoners comes as Ukrainian authorities face scrutiny this week for publicly displaying two men they say were Russian soldiers captured while fighting alongside separatists.
Amnesty is urging UN agencies and experts to visit detention sites in Ukraine to meet those being held by both sides.