WASHINGTON (AFP) - US President Barack Obama must investigate his predecessor George W. Bush and allies over the CIA's torture of terror suspects, or stand complicit in a government "cover-up," Human Rights Watch said on Tuesday (Dec 1).

In a scathing report, the international rights group decried the lack of prosecutions of those involved in the Central Intelligence Agency's secret programme to torture detainees in the wake of the Sept 11, 2001 attacks.

"While the programem officially ended in 2009, the cover-up of these crimes appears to be ongoing," said the report, which argues there is enough evidence for the attorney general to order criminal probes.

HRW cited Bush along with top officials including then CIA director George Tenet, former vice president Dick Cheney, former national security advisor Condoleezza Rice and former attorney general John Ashcroft.

Kenneth Roth, HRW's executive director, blasted Obama for his administration's failure to act, as the report was presented in Washington.

"Without criminal investigations, which would remove torture as a policy option, Obama's legacy will forever be poisoned," he said.

In the years following 9/11, the CIA "rendered" suspects to several countries where torture was tolerated and kept them locked up in secret prisons.

"Suspects basically disappeared and were subject to horrible brutality," Roth said. "Yet the Obama administration still has refused to act, to prosecute the senior officials who ordered or authorised the torture."

The 153-page report outlines evidence to support the main criminal charges that could be brought against those behind what so-called "enhanced interrogation" techniques, and challenges claims that prosecutions are impossible.

"US officials who created, authorised and implemented the CIA programme should be among those investigated for conspiracy to torture as well as other crimes," the report summary states.

In addition to members of the Bush administration, the report says James Mitchell and Bruce Jessen - who devised the torture program under a CIA contract - should also be investigated.

The American Civil Liberties Union in October sued the two psychologists on behalf of a group of former prisoners from the early days of America's "war on terror." The CIA's use of torture was detailed in an explosive Senate report released a year ago that describes the agency's interrogation of Al-Qaeda suspects, including beatings, "rectal rehydration" and sleep deprivation.

"Many detainees were held by the CIA in pitch-dark windowless cells, chained to walls, naked or diapered, for weeks or months at a time," recalled the HRW report.

"The CIA forced them into painful stress positions that made it impossible for them to lie down or sleep for days, to the point where many hallucinated or begged to be killed to end their misery," it said.

The level of mistreatment was more brutal than had been previously acknowledged - and did not produce useful intelligence.

"It's been a year since the Senate torture report, and still the Obama administration has not opened new criminal investigations into CIA torture," Roth said.

Human Rights Watch said that although much of the abuse took place a decade or more ago, it would still be open to prosecution.

"The statute of limitations for the crime of conspiracy may be extended if those responsible conceal a central component of the plot, which was the case with the CIA program," the group said in a statement.

Under the UN Convention against Torture, ratified by the United States in 1988, governments are meant to investigate allegations of torture.

"The failure to investigate and prosecute CIA torture increases the danger that some future president will authorize similar illegal interrogation methods in response to an inevitable serious security threat," the group said.

Roth said the United States cannot tell other countries to follow "the rule of law if they won't apply that same rule at home." Republican presidential hopeful Donald Trump recently called for the return of waterboarding, a simulated drowning interrogation technique widely denounced as torture, saying it was "peanuts" compared to what the Islamic State group is doing.