Washington  - CIA officials who used harsh interrogation techniques against terrorism suspects during the Bush administration will not be prosecuted, US President Barack Obama said Thursday.

CIA operatives who used harsh tactics authorized by then president George W Bush's administration were following the legal advice of the Justice Department and should not be held responsible, he said.

"We have been through a dark and painful chapter in our history," Obama said in a statement. "But at a time of great challenges and disturbing disunity, nothing will be gained by spending our time and energy laying blame for the past."

The White House also released memos drafted under the Bush administration that provided legal justification for the methods that included sleep deprivation and waterboarding, a technique used to simulate drowning that human rights groups say amounts to torture.

The CIA have previously acknowledged the waterboarding of three terrorism suspects, including Khalid Sheikh Mohammed, the alleged mastermind of the September 11, 2001, terrorist attacks.

"In releasing these memos, it is our intention to assure those who carried out their duties relying in good faith upon legal advice from the Department of Justice that they will not be subject to prosecution," Obama said.

Civil rights group ACLU welcomed the release but said it was still pushing for an independent prosecutor to investigate allegations of torture under the Bush administration.

"We have to look back before we can more forward as a nation," said Anthony Romero, executive director of the ACLU. "When crimes have been committed, the American legal system demands accountability."