A group of UN experts voiced alarm Monday at the widespread detention of immigrants and asylum seekers in the United States, and warned of "systemic problems" in the country's criminal justice system.

In a fresh report, the UN Working Group on Arbitrary Detention voiced concern over soaring detention rates as President Donald Trump's administration cracks down on illegal immigration and works to deport legal immigrants with criminal records.

"The Working Group is of the view that the mandatory detention of immigrants, especially asylum seekers, is contrary to international human rights and refugee rights standards," the five independent experts said.

The report, based on visits to a range of US detention facilities late last year, found that non-criminal immigrants were subjected to "punitive conditions that were often indistinguishable from those applicable to persons who had been sentenced to punishment in the criminal justice system".

The report also highlighted the difficulty for detained immigrants to access legal representation, unreasonably long incarceration -- including of children -- and the separation of family members during detention.

"Immigration detention should be the exception rather than the rule," the experts said, insisting that the current "excessive use of immigration-related detention" could not be justified by necessity.

They warned that current system "appeared to be implemented to deter individuals from continuing their immigration claims and could result in asylum seekers revoking their legitimate immigration claims."

Monday's report also examined the broader rights situation in US jails and prisons, pointing to "several areas where systemic problems in the criminal justice system are resulting in the arbitrary deprivation of liberty."

The panel listed lengthy pretrial detention, the lack of effective legal representation, economic and racial disparities, disproportionate sentences and the practice of treating minors in certain cases as adults.

They voiced concern that some states are increasing their use of the death penalty, especially in light of the problems they had seen in guaranteeing fair trials, arguing: "Imposing the death penalty after a trial at which the defendant was not afforded all necessary safeguards constitutes arbitrary deprivation of life".

The experts also decried the continued operation of the US military prison at Guantanamo Bay in Cuba, urging Washington to close the facility.

Former president Barack Obama promised to do so, but his successor Donald Trump has signalled he instead plans to ramp up use of the prison.

Source: AFP