An inmate at the U.S. detention facility at Guantanamo Bay is still being tortured, the UN special rapporteur on torture Nils Melzer said in a statement Wednesday.

Former President Barack Obama ended the use of "enhanced interrogation techniques" with an executive order in January 2009, but Melzer said the historic use of torture in CIA custody had not yet led to prosecutions or compensation for victims.

"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.

The statement cited the case of Ammar al-Baluchi, an inmate at the U.S.-controlled centre at Guantanamo Bay, Cuba, "where his torture and ill-treatment are reported to continue."

It said al-Baluchi was named 153 times in the U.S. Senate's 2014 "torture report," the result of a six-year investigation into so-called enhanced interrogation techniques used by the CIA after the Sept. 11, 2001, attacks.

He was said to have suffered relentless torture for 3½ years at CIA "black sites" before being moved to Guantanamo, the UN statement said.

It did not give details of the source of the information about the continuing torture of al-Baluchi, who had been in isolation at a severely restricted-access facility at Guantanamo Bay for more than a decade.

"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."

Major Ben Sakrisson, a Pentagon spokesman, offered a strenuous denial.

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

Melzer 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.

There are just over 40 detainees being held at the facility opened by George W. Bush after 9/11. Several were deported in the final years of Obama's second term. President Donald Trump, in contrast to Obama, has expressed enthusiasm for holding militants charged in attacks at Guantanamo as opposed to prosecuting them through U.S. criminal courts, exemplified by comments he made after a deadly New York City attack several weeks ago.

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."

Al-Baluchi, from Yemen, is the nephew of 9/11 plotter Khalid Sheikh Mohammed. He was among those detainees whose artwork was recently displayed at a midtown Manhattan gallery, the result of a Department of Defence initiative established in 2009 to help prevent further radicalization.