WASHINGTON—President Barack Obama Thursday blew the lid on harsh CIA terror interrogations approved by ex-president George W. Bush, including the use of insects, simulated drowning and sleep deprivation.

But despite releasing four partially blacked-out memos detailing the tactics, Obama said operatives who carried out the interrogations would not be prosecuted, saying they acted on orders and were defending their country.

"This is a time for reflection, not retribution," Obama said.

"We have been through a dark and painful chapter in our history. 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 graphic memos offered a stunning glimpse inside the covert interrogation program introduced after the September 11 attacks in 2001, which critics say equated to torture, and Obama said undermined America's moral authority.

The documents were written by Bush administration legal officials and argued that a long list of coercive techniques did not equal torture as they did not amount to the inflicting severe mental or physical pain.

Detailing methods used to question Al-Qaeda terror suspects, the memos reveal the use of dietary manipulation, forced nudity, facial and abdominal slaps, and the use of confined or "stress positions" for suspects.

In one technique known as "walling," interrogators could push a suspect against a false wall, so his shoulder blades make a slamming noise and make him think the impact is greater than in reality.

The memos also show interrogators asked for a ruling on whether the placing of a harmless insect in a cramped box with Al-Qaeda terror suspect Abu Zubaydah equated to torture.

The technique "certainly does not cause physical pain" and therefore could not be termed as torture and should be permissible, one of the memos said.

Similarly, techniques included waterboarding or simulated drowning, walling and sleep deprivation also fell short of torture, the memos said.

Another memo details a 'prototypical interrogation,' which begins with a detainee stripped of his clothes, shackled, and hooded, "with the walling collar over his head and around his neck."

Human rights groups reacted with dismay to Obama's decision to shield interrogators from prosecution.

"The Department of Justice appears to be offering a get-out-of-jail-free card to individuals who, by US Attorney General Eric Holder's own estimation, were involved in acts of torture," said Larry Cox, executive director of Amnesty International.

The Center for Constitutional Rights said the immunity for officials was "one of the deepest disappointments" of the Obama administration.

"Government officials broke very serious laws: for there to be no consequences not only calls our system of justice into question, it leaves the gate open for this to happen again."

In a statement, Obama said the tactics adopted by the administration of his predecessor "undermine our moral authority and do not make us safer."

He said he was releasing the documents to avoid "an inaccurate accounting of the past," which would "fuel erroneous and inflammatory assumptions about actions taken by the United States."

"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," he said in a statement.

"The men and women of our intelligence community serve courageously on the front lines of a dangerous world," he said.

Dennis Blair, the Director of National Intelligence said the torture memos must be read in the context of the "horror" of the September 11 attacks.

"Those methods, read on a bright, sunny, safe day in April 2009, appear graphic and disturbing," he said, but pledged Washington would not use similar methods in future.

"But we will absolutely defend those who relied on these memos and those guidelines," he said.

A federal court had given the government until Thursday to either turn over the memos in response to a lawsuit brought by the American Civil Liberties Union or explain why they cannot be released.

The memos were authored by Jay Bybee and Steven Bradbury, who at the time were lawyers for Bush's Justice Department Office of Legal Counsel.