GENEVA (Reuters) - An independent U.N. human rights investigator said on Wednesday that he had information about an inmate being tortured at the U.S. Guantanamo Bay detention facility, despite Washington banning “enhanced interrogation techniques” almost 10 years ago.

The U.S. Department of Defense denied the allegation, saying there was no credible evidence to support it.

Nils Melzer, the U.N. special rapporteur on torture, said he had information that Ammar al-Baluchi - accused of being a co-conspirator in the 9/11 attacks on the United States - was being subjected to treatment that is banned under international law.

“His torture and ill-treatment are reported to continue,” a statement from the U.N. human rights office said, without giving details of the source of Melzer’s information.

“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,” it said.

Major Ben Sakrisson, a Pentagon spokesman, said the allegation was not true.

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

FILE PHOTO - Chain link fence and concertina wire surrounds a deserted guard tower within Joint Task Force Guantanamo's Camp Delta at the U.S. Naval Base in Guantanamo Bay, Cuba March 21, 2016. REUTERS/Lucas Jackson/File Photo

The prison, which was opened by President George W. Bush to hold terrorism suspects captured overseas after the Sept. 11, 2001 attacks came to symbolize harsh detention practices that opened the United States to accusations of torture.

His successor Barack Obama ended the use of “enhanced interrogation techniques” via executive order in January 2009, and reduced the inmate population to 41, but fell short of fulfilling his promise to close the jail.

President Donald Trump asked Congress earlier this year for funds to upgrade the jail, having said during his electoral campaign that he wanted to “load it up with some bad dudes”.

Citing a 2014 Senate investigation, the U.N. statement said al-Baluchi was said to have suffered relentless torture for three-and-a-half years in CIA “black sites” before being moved to Guantanamo, where he had been in a severely restricted-access facility at Guantanamo Bay for more than a decade.

Al-Baluchi, a Kuwaiti-born Pakistani citizen also known as Abdul Aziz Ali, is the nephew and alleged co-conspirator of Khalid Sheikh Mohammed, the accused mastermind of the Sept. 11, 2001, attacks.

IMPUNITY

Melzer said the ban on torture and ill-treatment was one of the most fundamental norms of international law and could not be justified in any circumstances, and called for prosecution of U.S. officials who had carried out torture.

“By failing to prosecute the crime of torture in CIA custody, the U.S. is in clear violation of the Convention against Torture and is sending a dangerous message of complacency and impunity to officials in the U.S. and around the world,” Melzer said in the statement.

He said he had renewed a long-standing request to visit Guantanamo Bay to interview inmates, but he and his predecessors in the role had consistently been denied access.

A spokesman for the U.S. State Department said that the U.S. constitution prohibits cruel and unusual punishment and requires humane conditions of confinement, including that of solitary confinement.

“We support the work of the U.N. special rapporteurs and the United States has a long history of engaging constructively on matters within mandates of the special rapporteurs,” he said.