New York, New York - With nearly constant surveillance, grueling isolation, and limited family access, the treatment of the last 30 Guantánamo detainees is "cruel, inhuman,and degrading," UN rights experts said Monday as they reported on their first visit to the US military prison.

UN Special Rapporteur Fionnuala Ni Aolain said mistreatment at the prison on an American naval base in Guantánamo Bay, Cuba, amounted to violations of detainees' fundamental rights and freedoms.

The detainees, held close to two decades after being seized as suspects following the 2001 Al-Qaeda attack on the United States, have endured a litany of abuse, including forced cell extractions, poor medical and mental health care, said Ni Aolain.

The detainees also have had inadequate access to family either by in-person visits or calls, she said in a press briefing.

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"The totality of all of these practices and omissions... amounts in my assessment to ongoing cruel, inhuman, and degrading treatment under international law," she said.

Ni Aolain, the UN Special Rapporteur on the Promotion and Protection of Human Rights and Fundamental Freedoms while Countering Terrorism, travelled to Guantánamo with a team in February after UN rights experts had sought to visit the prison for two decades.

Introducing the team's report, she said Washington had yet to address the most glaring rights violation related to the detainees: their secret seizure and transfer – or "rendition" – to Guantánamo in the early 2000s, and, for many, enduring extensive torture by US operatives in the first years after the September 11 attacks.

Their planned military trials have been stalled for years as the question remains whether they can receive fair justice if they have been tortured.

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That is unfair to the victims of the September 11 attack as well, said Ni Aolain.

"The systematic rendition and torture at multiple (including black) sites and thereafter at Guantánamo Bay, Cuba... comprise the single most significant barrier to fulfilling victims' rights to justice and accountability," the UN Special Rapporteur said.

Ni Aolain welcomed the openness showed by President Joe Biden's administration in allowing her team to visit Guantánamo and examine the treatment of the detainees, who once numbered nearly 800. Still, the closure of the prison, which is outside the US justice system, "remains a priority."

In addition, "the US government must ensure accountability for all violations of international law, both for victims of its counterterrorism practices, present and former detainees, and victims of terrorism," she said.

Accountability, she said, includes apologies, full remedy and reparations for "all victims," she said.

In a letter to Ni Aolain on the report, Michele Taylor, the US envoy to UN Human Rights Council in Geneva, said the US does not accept all of her assessments.

"We are committed to providing safe and humane treatment for detainees," Taylor wrote, without offering any details or explanations.