It is no secret that the US military operates detention centres around the world for the interrogation of terror suspects.
Protesters accuse the US of condoning torture
The treatment of prisoners in these places - including Guantanamo Bay in Cuba, Bagram Air Base in Afghanistan and Abu Ghraib in Iraq - has come in for intense scrutiny and evidence of human rights violations has been widely reported.
But less well-documented is the process by which terror suspects are sent by the United States for interrogation by security officials in other countries.
This is known as "rendition" and is becoming increasingly controversial because many of these countries - including Syria and Egypt - are accused of using torture on prisoners, not least by the US State Department.
'Tortured in Syria'
Maher Arar is a Canadian citizen who in 2002 was detained in transit at New York's JFK airport, and accused of being an al-Qaeda member.
After 12 days in US custody, he was bundled in chains aboard a plane to Jordan and then taken by road to the Syrian capital, Damascus. There, Mr Arar claims, he was tortured by Syrian security police.
"The interrogator said, 'Do you know what this is?' I said, 'Yes, it's a cable,'" Mr Arar told the BBC.
"He told me: 'Open your right hand.' I opened my right hand and he hit me like crazy. And the pain was so painful and of course I started crying, and then they asked me questions."
Steven Watt from the American Civil Liberties Union has been closely involved in the case.
"The US State Department specifically states that the state security police in Syria torture and abuse detainees in their custody and that's exactly what happened to Maher," he said.
"In the first two weeks in particular he was beaten severely using electric cables, on all parts of his body, and he was detained in what he described as a grave. It was six feet [1.8m] long, three feet wide and six feet high and that was his home for some 10 months."
'Dozens of cases'
The Syrian authorities have confirmed that they did interrogate Mr Arar in relation to terrorist activities, but deny that any torture took place.
The case is far from unique. Human rights lawyers have documented similar stories from prisoners transferred to a range of countries which the US State Department recognises as routinely carrying out torture in detention - including Syria and Egypt.
Mr Watt says it is difficult to find out how many cases of rendition there have been by the US authorities.
"Well, it's highly sensitive, but just in recent months there have been reports of some 100 to 150 individuals who have been rendered in such fashion - that's since 9/11.
"Recently in an interview on US television [Egyptian President] Hosni Mubarak said 50 to 60 individuals alone had been rendered by the US to Egypt, so I think 100 to 150 is a fairly conservative estimate."
The US does not deny that terror suspects have been transferred in this way, but strongly rejects accusations that they are being tortured.
"In a post-9/11 world the United States must make sure we protect our people and our friends from attack," said President George W Bush when challenged on the issue at a press conference in March.
"That was the charge we had been given. And one way to do so is to arrest people and send them back to their country of origin, with the promise that they won't be tortured.
"That's the promise we receive. This country does not believe in torture. We do believe in protecting ourselves. We don't believe in torture."
The US authorities say they receive "diplomatic assurances" that those they transfer will not be tortured.
Stephen Grey, a journalist who has closely followed US rendition policy, is not convinced that this amounts to anything more than window-dressing for a highly controversial policy.
"Although the Bush administration is now saying they wouldn't be tortured - people who've actually been involved in this programme know full well what kind of countries they're dealing with," he told the BBC.
Mr Grey says Michael Scheuer, the former head of a CIA unit set up to track al-Qaeda chief Osama bin Laden, "has said that he told his bosses in the administration and the CIA that people they were sending to countries like Egypt would be tortured".
The US State Department declined a request for an interview, but did provide a statement.
"It is the long-standing policy of the United States not to transfer a person to a country if it determines that it is more likely than not that the person will be tortured," the statement says.
"Prior to any transfer, the department seeks assurances in every case in which continued detention by the government concerned is foreseen, of humane treatment and treatment in accordance with international obligations."
'Times of war'
Danielle Pletka is a vice-president of the American Enterprise Institute, a think-tank in tune with the politics of the Bush administration.
"I'm not a big fan of torture. Unfortunately, there are times in war when it is necessary to do things in a way that is absolutely and completely abhorrent to most good, decent people," she told the BBC.
"I don't want to say that the United States has engaged routinely in such practices, because I don't think that it is routine by any standard.
"But that said, if it is absolutely imperative to find something out at that moment, then it is imperative to find something out at that moment, and Club Med is not the place to do it."
But many, including the journalist Mr Grey, believe that aside from the human rights issues such intelligence gathering tactics simply do not produce the desired results.
"There are people inside the system, intelligence officers from America and the UK, who are unhappy with what's going on," he said.
"And they're not unhappy because they're soft on terrorism. They're unhappy because they think the whole thing is counter-productive.
"The kind of intelligence they get from torturers beating information out of people is often useless and it just feeds the whole intelligence system with all kinds of useless information that they spend years tracking down and get nowhere."
There is no doubt the sharing of intelligence between countries plays an essential part in defeating global terrorism.
But despite increasing public disquiet about both the morality and effectiveness of interrogation methods being used on terror suspects, neither President Bush nor the US State Department give the impression that the policy of rendition is currently under review.