Geneva, Switzerland - UN experts on Wednesday condemned the deportations of more than 250 Venezuelans and Salvadorans from the US to El Salvador, accusing Washington of denying them due process.

President Donald Trump's administration has paid El Salvador millions of dollars to lock up scores of migrants it says are criminals and gang members, in a maximum security prison with a history of alleged human rights violations.

Trump invoked an antiquated American law, the Alien Enemies Act of 1798, to defend his order to expel Venezuelans to El Salvador without trial, accusing them of belonging to the Venezuelan gang Tren de Aragua.

But nearly 20 independent United Nations rights experts charged in a statement Wednesday that "in many cases, the Alien Enemies Act of 1798 seems to have been misapplied to deny due process, independent review and court access, contrary to international human rights law".

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The experts, who are mandated by the UN Human Rights council but who do not speak on behalf of the United Nations, charged that "the lack of due process resulted in arbitrary deportation decisions against many people who were reportedly not involved with gangs".

They questioned the legal criteria for applying the act.

"There has plainly been no 'invasion' or 'predatory incursion' of the US by any foreign State, as required by the Act," they said, adding that "even if some individuals were gang members, gang activity is a crime, not an act of war".

The experts, including the special rapporteurs on torture and on the promotion and protection of human rights while countering terrorism and the working group on enforced disappearances, decried "the summary nature" of the deportation decisions.

These were "clearly inadequate to determine if people were at risk of serious human rights violations in El Salvador", they said.

They expressed grave concern about conditions in El Salvador prisons, pointing to reports of deaths, torture, and ill-treatment in custody, and violence between prisoners and enforced disappearances.

"Prison conditions are allegedly inhumane, contrary to international standards, with detainees held in overcrowded, boxed cages, without bare necessities such as mattresses, and with inadequate medical care," the statement said.

The experts expressed alarm at the unclear legal basis for the detention of the deportees in El Salvador, and their lack of access to legal representation, family visits, and judicial review.

"These rights must be immediately guaranteed," they said.