Critics say the methods approved in the memos amount to torture
CIA agents who used harsh interrogation techniques on terrorism suspects during the Bush era will not be prosecuted, US President Barack Obama has said.
The assurance came as memos were released detailing the range of techniques the CIA was allowed to use during the Bush administration.
Mr Obama banned the use of methods such as sleep deprivation and simulated drowning in his first week in office.
But rights groups have criticised the decision not to seek prosecutions.
Amnesty International said the Department of Justice appeared to be offering a "get-out-of-jail-free card" to individuals who were involved in acts of torture.
The Centre for Constitutional Rights, which has championed the legal rights of the "war on terror" detainees, also expressed its disappointment.
Aimed at simulating sensation of drowning. Used on alleged 9/11 planner Khalid Sheikh Mohammed
Harmless insect to be placed with suspect in 'confinement box', suspect to be told the insect would sting. Approved for Abu Zubaydah, but not used
Detainee slammed repeatedly into false wall to create sound and shock
Detainee shackled stading up. Used often, once for 180 hours
"It is one of the deepest disappointments of this administration that it appears unwilling to uphold the law where crimes have been committed by former officials," it said in a statement.
However, the former head of the CIA under former President George W Bush, Gen Michael Hayden, said the White House move would undermine intelligence work and dissuade foreign agencies from sharing information with the CIA.
"If you want an intelligence service to work for you, they always work on the edge. That's just where they work," he told the Associated Press.
The Obama administration did not say that protection would extend to CIA agents who acted outside the boundaries laid out in the memos, or to those non-CIA staff involved in approving the interrogation limits.
That leaves open the possibility that those lawyers who crafted the legal opinions authorising the techniques, one of whom is now a federal judge, could yet face legal action.
But the BBC's North America editor Justin Webb, in Washington, says it seems that the Obama administration does not want any prosecutions and would like the matter closed.
The Obama administration said the move reiterated its previously-stated commitment to end the use of torture by its officers, and would protect those who acted within the limits set out by a previous legal opinion.
Announcing the release of the four memos, Attorney General Eric Holder said the US was being "consistent with our commitment to the rule of law".
"The president has halted the use of the interrogation techniques described in these opinions, and this administration has made clear from day one that it will not condone torture," he said.
The four secret memos detail the legal justification for the Bush-era CIA interrogation programme, whose methods critics say amounted to torture.
Mr Obama gave an assurance that "those who carried out their duties relying in good faith upon legal advice from the Department of Justice... will not be subject to prosecution".
One of the documents contained legal authorisation for a list of specific harsh interrogation techniques, including pushing detainees against a wall, facial slaps, cramped confinement, stress positions and sleep deprivation.
The memo also authorises the use of "waterboarding", or simulated drowning, and the placing of a detainee into a confined space with an insect.
Critics of the Bush-era interrogation programme say the newly-released memos provide evidence that many of the methods amount to torture under US and international law.
"Bottom line here is you've had crimes committed," Amnesty International analyst Tom Parker told the BBC.
"These are criminal acts. Torture is illegal under American law, it's illegal under international law. America has an international obligation to prosecute the individuals who carry out these kind of acts."
Mr Parker said the decision to allow the use of insects in interrogation was reminiscent of the Room 101 nightmare described by George Orwell in his seminal novel, 1984.
HAVE YOUR SAY
Punishment is a deterrent against future transgressions
The approved tactic - to place al-Qaeda suspect Abu Zubaydah, who is afraid of insects, inside a box filled with caterpillars but to tell him they were stinging insects - was never used.
Despite that, the memo was "incredibly depressing reading if you're somebody who loves America", Mr Parker said.
During his first week in office, President Obama issued an executive order officially outlawing the use of harsh interrogation techniques by the CIA, and forcing the agency to adhere to standards laid out in the US Army Field Manual.
The release of the memos stems from a request by civil rights group the American Civil Liberties Union (ACLU).
REACTIONS TO OBAMA DECISION
The memos' matter-of-fact clinical descriptions belie the harsh tactics to which they gave a green light. But... it is not enough to say that when we have a president who does not believe in cruel and inhuman treatment and torture, the United States will not engage in such practices. We must formally acknowledge that what was done was wrong, indeed criminal.
Georgetown University Professor David Cole, debating the issue at
The New York Times
By repudiating the memos, the Obama administration has again seized the high ground and restored some of the honor lost over the past few years. Yet the decision to forgo prosecutions should not prevent -- and perhaps should even encourage - further investigation about the circumstances that gave rise to torture.
The Washington Post
On the surface, the statement today looks like a big ol' grant of immunity - or a concession - or a deliberate attempt to boost morale at the CIA... There are plenty of CIA officers who followed the rules and shouldn't be prosecuted. They're the ones who are a little relieved today... although they might have to explain some things to their priests and their families.
Another major issue is lingering, however. Did the torture "work"?... Without a rigorous investigation into the alleged efficacy of U.S. torture, we'll never know. But while Obama has turned the page, many others haven't - including the people, and their allies, who think waterboarding was a good idea. Without a commission... we could start torturing all over again.