GENEVA: The office of the U.N. human rights chief is decrying the "mass hanging" of 42 prisoners at an Iraqi prison.

Zeid Ra'ad al-Hussein is warning of "clear risk of a gross miscarriage of justice" over Sunday's executions at al-Hoot prison in the southern city of Nasiriyah, and expressed concerns about "more large-scale executions," possibly in coming weeks.

A statement from his office Wednesday said Iraqi officials explained the prisoners were Iraqis affiliated with the Islamic State group or al-Qaida who were convicted of crimes including killing security forces or detonating improvised explosive devices.

Zeid said accountability for proven acts of terrorism was necessary, but it was "extremely doubtful" that strict fair-trial guarantees were respected in the cases as required under international law for the death penalty to be applied.