The techniques have been authorised for use at CIA "black sites" abroad, at which top terror suspects are held. Last week the US-based organisation Human Rights Watch said "ghost detainees" were held at two military bases, in Poland and Romania. Similar sites in half a dozen other countries, including Afghanistan, Thailand and the Indian Ocean base of Diego Garcia, leased from Britain, are now said to have been closed.

The existence of these detention facilities, and what happens inside them, are the most secret aspect of America's "war on terror". In contrast to military-run camps and prisons such as Guantanamo Bay in Cuba or Abu Ghraib in Iraq, where it was impossible to shield all CIA activity from outside scrutiny, the location of the "black sites" and the identities of those held there are made known only to a handful of senior officials in the US. In the host countries, only the president and top intelligence officials are aware of them.

Details of the secret prisons and the methods used in them have emerged mainly from CIA officers themselves, who said the public needed to know "the direction their agency has chosen". They broke ranks amid a furore in Washington over an amendment to the White House military spending package going through Congress. Senator John McCain (Republican), a former US navy pilot who was captured and tortured in Vietnam, wants an unequivocal ban on all "cruel and inhuman" treatment of prisoners in US custody, including those held by the CIA.

Eighty-nine of Mr McCain's fellow senators voted for his amendment, rejecting attempts by the CIA and Vice-President Dick Cheney - who said after 9/11 that "we have to work ... the dark side" - to exclude prisoners held at the "black sites". For the first time President George Bush has threatened to exercise his veto on any defence bill that has the amendment attached.

The CIA prisons contain only the 30 or so most senior al-Qa'ida captives. They include Khalid Sheik Mohammed, the alleged mastermind of the September 2001 attacks, Ramzi Binalshibh, another prime 9/11 suspect, and the Indonesian Riduan Isamuddin, better known as Hambali, accused of masterminding the Bali nightclub bombings in October 2002. Only the merest hints have emerged about their treatment, but according to the most graphic account, given to ABC News in the US, Khalid Sheik Mohammed won the admiration of his interrogators by enduring "waterboarding" for up to two and a half minutes before begging to confess. CIA officers who subjected themselves to the same technique lasted an average of 14 seconds.

ABC's sources said that just over a dozen CIA interrogators were trained and authorised to use the "enhanced interrogation" techniques. At least three had declined involvement. The use of each technique on each prisoner had to be approved, stage by stage, up to the use of the "water board". About a dozen "high-value" al-Qa'ida targets had been interrogated in this way, and, as one put it: "All of these have confessed, none of them has died, and all of them remain incarcerated."

At least one death has been reported elsewhere, however. In a CIA facility in Kabul known as the "Salt Pit", an officer, described as young and inexperienced, used the "cold treatment" on a detainee, who was left outdoors, naked, throughout a freezing Afghan night. He died of hypothermia. The case is being investigated, along with several others in Afghanistan and Iraq where interrogators - CIA officers, civilian contractors or members of the special forces - went well beyond the guidelines and suspects died as a result.

Colonel Lawrence Wilkerson, chief of staff to Colin Powell when he was US Secretary of State, said last week that he knew of more than 70 "questionable deaths" of detainees under US supervision up to the end of 2002, when he left office. That figure, he added, was now around 90.

These incidents are in addition to the increasingly well-documented practice of "rendition": flying suspects to Middle Eastern countries where torture and deaths in custody are routine. "If you want a good interrogation, you send them to Jordan. If you want them dead, you send them to Egypt or Syria," one former CIA agent is reported as saying. The McCain amendment, however, will have no impact on foreign torturers. It is mainly aimed at halting the abuses exposed at Abu Ghraib, where routine humiliations degenerated into sadism.

Yet only the low-ranking military police caught on camera in Abu Ghraib have been prosecuted. America's covert forces are operating in a climate of impunity, described by Cofer Black, then CIA counter-terrorism chief, who told a congressional committee in 2002: "After 9/11, the gloves were off." At one point, according to Newsweek, the Bush administration formally told the CIA it could not be prosecuted for any technique short of inflicting the kind of pain that accompanies organ failure or death.

The normal justification is that such methods could help avert a terror attack in which thousands might be killed. But are there any cases to prove it? Claims that the "waterboarding" of Khalid Sheik Mohammed produced details of planned attacks on the US were sceptically received by one CIA official, who said: "What we got was probably not truthful. And there's no way of knowing whether what good information they got could not have been obtained by more traditional means."

THE CIA'S SIX 'ENHANCED' TECHNIQUES

CIA interrogators say they are allowed to use six "enhanced interrogation techniques", each progressively tougher, on top al-Qa'ida detainees. Their superiors have to give separate approval for every prisoner and every method, all claimed to be legal.

THE ATTENTION GRAB: The interrogator forcefully grabs the shirt front of the prisoner and shakes him. Israel, the only democracy to have openly debated coercion of prisoners, declared this legal in 1987, but the Supreme Court ruled it out in 1999

THE ATTENTION SLAP: Interrogators may deliver "an open-handed slap", which is "aimed at causing pain and triggering fear"

THE BELLY SLAP: A hard open-handed slap to the stomach. The aim is to cause pain, but not internal injury. Doctors advised against using a punch, which could cause lasting internal damage

STANDING FOR HOURS: This technique is described as among the most effective. Prisoners are forced to stand, handcuffed and with their feet shackled to a ring bolt in the floor for more than 40 hours. Exhaustion and sleep deprivation are claimed to be effective in yielding confessions

COLD TREATMENT: The prisoner is left to stand naked in a cell kept at around 10C, and constantly doused with cold water. Misapplication of this technique is blamed for the death of a detainee in Kabul

WATERBOARDING: The prisoner is bound to a board, head slightly below the feet. Plastic is wrapped over his face and water is poured over him, or his head is lowered into a bath. The gag reflex is automatic; few can endure more than a matter of seconds