US President George W. Bush has designated six al-Qaeda suspects as eligible to be tried before US military commissions.
"The president determined that there is reason to believe that each of these enemy combatants was a member of al Qaeda or was otherwise involved in terrorism directed against the United States," the Pentagon said in a statement.
Defence officials refused to identify the six individuals by name or even by nationality, saying no charges have yet been brought against them, and that their identities may never be announced.
The six are believed to be among the more than 600 prisoners imprisoned at the US naval base at Guantanamo Bay, Cuba. Any trials are set to be held at Guantanamo. The accused could face the death penalty.
Human rights groups said the rules for the trials set by the Pentagon are biased toward the prosecution, place unacceptable conditions on the defence and allow for no independent judicial review by civilian courts.
Charges set out in the Pentagon's instructions for the trials could bring the death penalty.
Officials said there was evidence the six had attended "terrorist" training camps and may have been involved in financing Saudi-born Osama bin Laden's al-Qaeda network, providing protection for bin Laden and recruiting future members.
Mr Bush may designate additional captives as eligible for trial, defence officials added.
Prisoners from Afghanistan and dozens of other countries have been held at the Guantanamo prison, established in January 2002, without charges and without access to lawyers. Once charges are brought, a defendant is due to be assigned a military defense lawyer and has the right to hire a civilian lawyer approved by the Pentagon.