Human rights groups have expressed concern following confirmation that six men accused of organising the 11 September 2001 attacks on the US will face military trial.
Prosecutors say they will be seeking the death penalty against the defendants, who are being held at the Guantanamo Bay camp in Cuba.
However groups such as Amnesty International say they fear evidence may be used that was obtained by torture, including practices such as 'waterboarding'.
The simulated-drowning interrogation technique inflicted on US terrorism suspects has become a focus for debate.
Last week, UN human rights chief Louise Arbour insisted waterboarding should be prosecuted as torture.
Civil rights advocates and terrorism analysts said its use on Khalid Sheikh Mohammed, the alleged mastermind of the attacks, undermines the credibility of any verdict.
'The government has now placed itself in a serious bind, first by subjecting him to waterboarding and other harsh interrogation techniques that the world regards as torture, and now by trying him in a military commission that allows the use of coerced testimony,' said Steven Shapiro of the American Civil Liberties Union.
Despite concerns by human rights organisations, US Homeland Security Secretary Michael Chertoff has said the six accused will get a fair trial.
He made his remarks after the US Defence Department announced that it was bringing formal charges of murder and conspiracy against the men.
The UN special rapporteur on torture, Manfred Nowak, said the six men could become the victims of a show trial.
