The top human rights official at the United Nations rang alarm bells Tuesday about the Trump administration’s use of a Salvadoran megaprison to detain immigrants without due process, potentially for life.

U.N. High Commissioner for Human Rights Volker Türk said in a press release that his office had received information from family members and lawyers “regarding more than 100 Venezuelans believed to be held in CECOT,” or Centro de Confinamiento del Terrorismo, the Salvadoran megaprison with a reputation for torturous conditions where the Trump administration has sent at least 288 men without charge or trial.

“This situation raises serious concerns regarding a wide array of rights that are fundamental to both US and international law — rights to due process, to be protected from arbitrary detention, to equality before the law, to be protected from exposure to torture or other irreparable harm in other States, and to an effective remedy,” the human rights commissioner said.

Türk said the sense of “powerlessness” experienced by families of detainees seeing their loved ones treated like terrorists even without trial, as well as the use of shackles and “demeaning rhetoric” by the Trump administration, have been “profoundly disturbing.”

“I welcome the essential role that the US judiciary, legal community and civil society are playing to ensure the protection of human rights in this context,” he said.

“I have called on the US Government to take the necessary measures to ensure compliance with due process, to give prompt and full effect to the determinations of its courts, to safeguard the rights of children, and to stop the removal of any individual to any country where there is a real risk of torture or other irreparable harm,” he added.

Trump’s agreement with the government of El Salvador to detain hundreds of immigrants sent by the United States has raised both domestic and international legal challenges.

Last week, human rights groups petitioned the Inter-American Commission on Human Rights on behalf of over a dozen detainees and their families, and hundreds more men believed to be detained in the Salvadoran prison system, seeking emergency measures including the detainees’ release.

In the United States, courts have insisted that detainees be given the opportunity to contest their removals to so-called “third countries” — those other than the nations of their citizenship — and judges have paused removals under the Alien Enemies Act, one of the authorities Trump used to send detainees to CECOT.

Under that 18th-century law, migrants deemed by immigration authorities to be members of the Tren de Aragua gang were treated as enemy combatants and expelled to CECOT before being able to fully hear or dispute the allegations against them.

Neither the United States nor El Salvador have released the names of U.S. detainees in CECOT, leading many human rights experts to classify the detentions as enforced disappearances.

“Given the risk, the fear, of human rights violations being committed, there may well be concerns of enforced disappearance,” Liz Throssell, a spokesperson for the U.N. human rights office, said at a press conference Tuesday.

“And of course that is a really a serious issue under international human rights law.”