Amnesty International has condemned the decision by the Pentagon to charge the alleged planner of the September 11th attacks and five others with murder and conspiracy and to seek their execution if convicted.
The charges are the first from the Guantánamo war court alleging direct involvement in the 2001 attacks on the United States and the first involving the death penalty.
In a statement today, Amnesty said the move raised "yet more" questions about US conduct in the "war on terror".
Amnesty claimed five of the six men charged were held for more than three years in secret CIA custody at unknown locations before being transferred to Guantánamo in September 2006.
The CIA has confirmed that at least one of the men, Khalid Sheikh Mohammed, was subjected to "waterboarding", an interrogation technique of simulated drowning that has been widely criticised as torture.
"Waterboarding is torture, and torture is an international crime. No one has been held accountable for such crimes. Impunity in relation to the CIA programme remains a hallmark of the USA's conduct in the 'war on terror,'" Amnesty said.
The charges must be approved by a Pentagon appointee who oversees the court before a trial can be ordered.
Military prosecutors want to try all six defendants together - Mohammed, Ali Abdul Aziz Ali, Mohammed al-Qahtani, Ramzi Binalshibh, Mustafa Ahmed al-Hawsawi and Walid bin Attash.
The Guantánamo tribunals are the first US war crimes tribunals since World War Two. They were established after September 11th to try non-American captives that the Bush administration considers "enemy combatants" not entitled to the legal protections granted to soldiers and civilians.
Brig Gen Thomas Hartmann, a legal adviser at the Guantanamo prison camp, said there will be no "secret" trials. "Every piece of evidence, every stitch of evidence, every whiff of evidence that goes to the finder of fact, to the jury, to the military tribunal, will be reviewed by the accused," he said.
Amnesty called on the international community to challenge the United States to drop the charges and for the men "to be tried in front of independent and impartial courts, without resort to the death penalty".