The UN's top expert on torture on Wednesday said reports from sources indicated at least one inmate was still being tortured at the US detention centre in Guantanamo Bay and reiterated his request to visit.

Nils Melzer, the UN special rapporteur on torture, said he had received information that torture using noise and vibrations was continuing against one of the suspected 9/11 plotters being held at Guantanamo.

He accused Washington of violating an international torture ban and also criticised the country for not holding US officials accountable for using abusive practices in the past.

Melzer pointed out that a US Senate Intelligence Committee report published three years ago acknowledged the Central Intelligence Agency's use of torture in the wake of the 9/11 attacks in 2001.

"To this day, however, the perpetrators and policymakers responsible for years of gruesome abuse have not been brought to justice, and the victims have received no compensation or rehabilitation," he said in a statement.

"By failing to prosecute the crime of torture in CIA custody, the US is in clear violation of the Convention against Torture," he said, warning that Washington was "sending a dangerous message of complacency and impunity to officials in the US and around the world."

Melzer voiced particular concern about detainees who had been held for long periods of time in almost complete isolation.

He highlighted the case of Ammar al-Baluchi, the nephew of alleged 9/11 mastermind Khalid Sheikh Mohammed, who himself stands accused of arranging the financing for the attacks that killed almost 3,000 people.

- Torture ban 'absolute' -

Al-Baluchi was named 153 times in the Senate report, and is said to have suffered "relentless torture" at CIA "black sites" for three and a half years before being moved to Guantanamo Bay, where the torture is reported to continue, Wednesday's statement said.

"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," he said.

The UN expert alleged that al-Baluchi was not receiving adequate medical attention for those disorders.

Melzer stressed that the international ban on torture and ill-treatment was "absolute" and "one of the most fundamental norms of international law."

"No circumstances, however exceptional and well-argued, may be invoked to justify torture," he insisted.

He urged Washington to "live up to its legacy, to end its policy of impunity and to bring its own perpetrators to justice."

The UN expert also reiterated a long-standing request to conduct an official visit to the Guantanamo Bay detention centre and to interview inmates.

"I very much regret that, despite repeated requests, my predecessors and I have consistently been refused access to Guantanamo," he said.

Former US President Barack Obama outlawed brutal interrogation techniques like water boarding, used by the CIA following the 9/11 attacks. He also attempted throughout his presidency to shutter Guantanamo.

But his successor Donald Trump has said he wants to send more "bad dudes" to Guantanamo and spoke positively about torture during his election campaign.

He has however since said he will follow the advice of his top aides, including Defense Secretary Jim Mattis, who opposes the practice.