A UN expert on Monday called on the U.S. government to apologize for the inhuman treatment of detainees at the Guantanamo Bay detention facility, which was established by former U.S. President George W. Bush's administration in 2002 following the September 11, 2001 attacks.

The UN Special Rapporteur on counter-terrorism and human rights, Fionnuala Ni Aolain, said the 30 men that are still being held there are subject to "ongoing cruel, inhuman and degrading treatment."

Ni Aolain's trip to Guantanamo Bay was the first official visit to the site by a UN expert.

"I want to acknowledge that there have been significant improvements made to the conditions of confinement at the Guantanamo Bay detention facility. That said, I still have serious concerns about the continued detention of now 30 men still there and the persistent arbitrariness that pervades their day-to-day existence," she said.

"My report details structural shortcomings and systematic arbitrariness, including in training, operating procedures, and the fulfillment of detainees' rights to health care, family, counsel, and justice."

The report sets out in detail how such arbitrariness brings about deep insecurity, suffering and anxiety for all detainees without exception, she said. "I observed that after two decades of custody, the suffering of those detained is profound and is ongoing."

"Every single detainee I met with lives with the unrelenting harms that follow from systematic practices of rendition, torture, and arbitrary detention... And their past experiences of torture live with them in the present without any obvious end in sight, including because they have not received any adequate torture rehabilitation to date," said Ni Aolain.

She said that despite the evident nature of the physical and psychological harms, the Guantanamo Bay detention infrastructure entails near-constant surveillance, forced cell extractions, undue use of restraints, and other arbitrary and human rights-noncompliant implementation of the standard operating procedures.

"The totality of all of these practices and omissions... have accumulative, compounding effects on the dignity and fundamental rights and freedoms of each detainee and amounts in my assessment to ongoing cruel, inhuman and degrading treatment under international law," she said.

The U.S. government must ensure accountability for all violations of international law, both for victims of its counter-terrorism practices, present and former detainees, and victims of terrorism, said Ni Aolain. "I underscore the importance of apology, full remedy, reparation and guarantees of non-repetition to all victims. And these guarantees will be no less pressing in the years ahead."

As for the 741 men who have been released from Guantanamo, she said, many were left on their own, lacking a legal identity, education and job training, adequate physical and mental health care, and continue to experience "sustained human rights violations," poverty, social exclusion and stigma.

Ni Aolain was appointed the special rapporteur on the promotion and protection of human rights and fundamental freedoms while countering terrorism by the UN Human Rights Council. She took up her functions in August 2017.

(With input from agencies)