BAGHDAD — A prominent rights group condemned Iraq on Tuesday for the mass execution of 21 people, some with suspected links to the Islamic State group, who had been convicted on terrorism charges.

The prisoners were hanged Monday in Nasiriyah Central Prison in southern Iraq, according to a semi-official provincial news site. Among them were men with suspected IS-links convicted under a 2005 counter-terrorism law. They had been charged with detonating explosives in the northern Iraqi town of Tal Afar during battles to dislodge the militant group, according to the report.

There was no official confirmation of the executions by the Iraqi government and the judiciary has not issued a statement. Iraqi President Barham Salih typically must approve such executions.

Rights groups have long criticized Iraqi court proceedings against IS-linked suspects as unfair, claiming individuals were sentenced to death in rushed trials with little evidence, and sometimes based on confessions extracted through torture.

Amnesty International in a statement called Monday's executions "an outrage."

"It is high time the Iraqi authorities put an end to executing people," said Lynn Maalouf, Amnesty's deputy regional director for the Middle East and North Africa. "Retaliatory executions not only fail to deliver justice to the victims and to their families, they serve to reinforce perceptions of partial justice."

U.N. High Commissioner for Human Rights Michelle Bachelet described the report of the executions as "deeply troubling," according to a statement from her office. The statement noted there has been no official confirmation of the executions, and she called the lack of transparency "unacceptable."

"I call on the Iraqi authorities to halt any further executions," said Bachelet. "I am deeply concerned about the fate of the several hundred prisoners who may be at imminent risk of execution in Iraq."