Detainees sit in a holding area at the US Navy base at Guantanamo Bay, Cuba, during in-processing to the temporary detention facility on Jan. 11, 2002. (AP)

The US treatment of the final 30 Guantanamo inmates is "cruel, inhuman, and degrading" with nearly constant surveillance, grueling isolation, and limited family access, UN rights experts said Monday in a report on their first visit to the detention center that still stands as one of the US many disgraceful stains and horrendous acts in history.

UN Special Rapporteur Fionnuala Ni Aolain said mistreatment at the prison on an American naval base in Guantanamo 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 9/11 attack on the United States, have endured a litany of abuse, including forced cell extractions and poor medical and mental healthcare, added Ni Aolain.

In a news conference, she acknowledged that the detainees' access to family members via phone calls or in-person visits has been insufficient.

"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.

A delegation led by Ni Aolain, the UN Special Rapporteur on the Promotion and Protection of Human Rights and Fundamental Freedoms while Countering Terrorism, visited Guantanamo in February after UN rights experts had sought to visit the prison for two decades.

She said while introducing the team's report that Washington had yet to address the violation of the most obvious rights involving the detainees: their covert seizure and transfer to Guantanamo in the early 2000s, and, for many of them, enduring extensive torture by US agents in the first years following the US alleged "war on terror."

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

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

She said the closure of the prison, which remains 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, according to her, includes apologies, full remedy, and reparations for "all victims".

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.

Earlier this month, a UN watchdog found the US and seven other nations liable for the torture and wrongful imprisonment of a Saudi Arabian prisoner awaiting a death sentence trial at the Guantanamo Bay detention facility.

The United Nations Working Group on Arbitrary Detention detailed that imprisonment at Guantanamo following the US alleged "war on terror" could be equivalent to crimes against humanity.

Guantanamo torture

The military prison in Cuba -- created after the September 2001 attacks to house detainees in the US so-called "war on terror"-- is still operating despite international and local calls to close it.

A group of UN experts has repeatedly urged Washington to finally close the site of "unrelenting human rights violations."

The US justified its torture of inmates by claiming that its campaign was targeted at finding "terrorists" responsible for the 9/11 attacks in order to prevent further attacks on US soil.

However, civil society groups have stated that many of Washington's actions, including rendition and "enhanced interrogation techniques," are unlawful under international law.