"I call on the Iraqi authorities to halt any further executions," UN High Commissioner for Human Rights Michelle Bachelet said in a statement.

"I am deeply concerned about the fate of the several hundred prisoners who may be at imminent risk of execution in Iraq," she added.

Her comments came a day after 21 Iraqi men were hanged at the notorious Nasiriyah prison in the country's south, medical and police sources told AFP.

The 21 had all been convicted under the country's 2005 Counter-Terrorism Law, which carries the death penalty, but no details were known about their specific crimes.

Nasiriyah prison in Dhi Qar province, the only one in Iraq that carries out capital punishment, is known for holding condemned ex-officials of the Saddam Hussein regime which was toppled by the 2003 US-led invasion.

"Our assessment of the administration of justice in terrorism-related cases in Iraq has found frequent violations of fair trial rights," Bachelet said.

She also complained that "ineffective legal representation, overreliance on confessions and frequent allegations of torture or ill-treatment" were common.

- 'Unacceptable' -

"In such circumstances, the implementation of the death penalty is of particular concern, and may amount to an arbitrary deprivation of life by the State," she said.

The UN rights chief pointed out that there had still been no official confirmation that Monday's executions took place, stressing that the lack of transparency in such a case was "unacceptable".

"The people of Iraq, including the victims of serious human rights violations and abuses, deserve justice," Bachelet said.

"But I fear that these executions may have the effect of compounding injustice."

Since declaring the Islamic State group defeated in late 2017, Iraq has condemned hundreds of its own citizens to death for membership in the jihadist faction.

But only a small proportion of the sentences have been carried out, as they must be approved by the country's president, currently Barham Saleh

Some of the men executed Monday were allegedly affiliated to IS, and Amnesty International's regional director in the Middle East, Lynn Maalouf, hinted their killings could be seen as retaliatory.

"Retaliatory executions not only fail to deliver justice to the victims and to their families, they serve to reinforce perceptions of partial justice," she warned in a statement.

This was particularly true, she said, "at a time when the authorities are mum in regards to other serious violations such as torture and enforced disappearances that are still taking place across the country".

Iraq ranks fifth among countries that carry out death sentences, according to Amnesty International, which documented 100 executions in the country in 2019.