US President Barack Obama has assured CIA agents involved in tough terrorism interrogations they will not be prosecuted.

'This is a time for reflection, not retribution,' Mr Obama said last night.

The president was releasing a series of internal memoranda from the Bush administration, in which Justice department lawyers argued that a long list of coercive techniques did not equal torture since they did not inflict severe mental or physical pain.

Mr Obama said the practices undermined the moral authority of the US.

Human rights groups reacted with dismay to his decision that interrogators will not face prosecution.

National Intelligence director Dennis Blair pledged that Washington would not use similar methods in the future.

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

Human Rights Watch welcomed the possibility that 'the president left open the possibility of prosecuting those higher up the chain who wrote the opinions and authorised the CIA to use abusive interrogation techniques and torture.'

The four memos represent the Bush administration's legal justification for interrogation methods that far exceeded US military limits and have been called torture by many countries.

Violating laws against torture is punishable by up to 20 years in prison, or if a person dies, by execution or life in prison.

The first memo, from 2002, approves waterboarding - a form of simulated drowning - and other harsh techniques on suspected high-level al-Qaeda figure Abu Zubaydah.

It was written by former Assistant Attorney General Jay Bybee for the CIA's top lawyer, John Rizzo. The CIA has confirmed that two other high-level al Qaeda suspects were waterboarded.

'We find that the use of the waterboard constitutes a threat of imminent death', one of the criteria for torture, the memo said. 'It creates in the subject the uncontrollable physiological sensation that the subject is drowning.'

However, it said, 'in the absence of prolonged mental harm, no severe mental pain or suffering would have been inflicted, and the use of these procedures would not constitute torture within the meaning of the statute.