Nils Melzer, the UN special rapporteur on torture, on Wednesday urged the United States to end its torture of detainees held at the controversial Guantanamo Bay detention facility.

The prison was opened under former President George W. Bush in the wake of the September 11, 2001, attacks for the purpose of housing alleged terror suspects. Although former President Barack Obama attempted to close the facility, it remains open, in part as a policy directive from sitting president, Donald Trump.

The UN, Western countries and human rights organizations have criticized US authorities for creating a "legal black hole" that allows for the indefinite detention of suspects without charge, and for holding many of the detainees for more than a decade.

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Continued torture

In his appeal, UN expert Melzer singled out the case of Ammar al-Baluchi, which was mentioned 153 times in a 2014 US Senate report on the Central Intelligence Agency's (CIA) detention and interrogation program that alluded to his continued torture.

Al-Baluchi, who stands accused of assisting the suspected perpetrators of the September 11 attacks, continues to be tortured even after receiving degrading treatment for more than three years at various CIA "black sites," Melzer said.

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"Mr. al-Baluchi has been held in isolation at a severely restricted-access facility at Guantanamo Bay for more than a decade," Melzer said. "In addition to the long-term effects of past torture, noise and vibrations are reportedly still being used against him, resulting in constant sleep deprivation and related physical and mental disorders, for which he allegedly does not receive adequate medical attention."

Trump's endorsement

Days before assuming office in January, Trump endorsed the use of torture against terror suspects, saying: "Absolutely I feel it works."

In 2014, he condemned then-President Obama and Democratic lawmakers for release a Congressional report outlining the CIA's controversial "enhanced interrogation" program.

"If America was under the threat of imminent attack, would Obama use torture or a kiss?" Trump said in a tweet.

'Dangerous message'

UN expert Melzer criticized the United States for its failure to prosecute those responsible for perpetrating practices of torture and other forms of degrading treatment.

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"By failing to prosecute the crime of torture in CIA custody, the US is in clear violation of the Convention against Torture and is sending a dangerous message of complacency and impunity to officials in the US and around the world," Melzer said. "This is one of the most fundamental norms of international law, and its violation is listed among the most serious international crimes, including crimes against humanity and war crimes."

Later on Wednesday, Pentagon spokesman Major Ben Sakrisson denied torture was still being practiced at the Guantanamo Bay facility.

"These claims have been investigated on multiple occasions in the past and no credible evidence has been found to substantiate his claims," Sakrisson said.