The United Nations voiced alarm at the large numbers of non-nationals being deported from the United States, in particular the hundreds sent to a mega-prison in El Salvador.

"This situation raises serious concerns regarding a wide array of rights that are fundamental to both US and international law," UN High Commissioner for Human Rights Volker Turk said in a statement on Tuesday.

His office pointed to US data showing that between January 20 and April 29, 142,000 individuals had been deported from the US.

It voiced particular concern at the situation after US President Donald Trump invoked the 1798 Alien Enemies Act in March to send alleged members of the Venezuelan gang Tren de Aragua to a massive prison, CECOT, in El Salvador.

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The US Supreme Court and several lower courts have since temporarily halted deportations using the obscure law, citing the lack of due process.

And yet, "the fate and whereabouts of at least 245 Venezuelans and some 30 Salvadorans removed to El Salvador remain unclear", the UN rights office said.

It said it had received information from family members and lawyers regarding more than 100 Venezuelans believed to be held in CECOT, where it decried reports of very harsh conditions.

"We are concerned of course about the conditions that they're being held in," UN rights office spokesperson Liz Throssell told reporters in Geneva.

"But we're really concerned about the fact that they're being held at all," she said, adding that the rights office had requested access to the prison.

'Enforced disappearances'

It cited reports indicate that "many of the detainees were not informed of the US government’s intention to deport them to be held in a third country, that many did not have access to a lawyer and that they were effectively unable to challenge the lawfulness of their removal before being flown out of the US".

It highlighted that to date, no official lists of the detainees had been published by US or Salvadoran authorities, and their legal status in El Salvador remains unclear.

"We don't know what is happening to people... Families do not know where their loved ones are. Lawyers do not know where they are. In fact, no-one knows where they are," Throssell said.

Turk stressed the anguish of the families of those taken away, who he said had expressed "a sense of complete powerlessness in the face of what has happened and their pain at seeing their relatives labelled and handled as violent criminals, even terrorists, without any court judgement as to validity of what is claimed against them".

"The manner in which some of the individuals were detained and deported -- including the use of shackles on them -- as well as the demeaning rhetoric used against migrants, has also been profoundly disturbing."

The UN rights chief said he welcomed "the essential role that the US judiciary, legal community and civil society are playing to ensure the protection of human rights in this context".

"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."