Amnesty International has called on the United States to either release the rest of the prisoners held at the U.S. Naval Base at Guantanamo Bay, Cuba, or try them fairly on criminal charges.
"It is time to end the unacceptable legal limbo in which these prisoners are kept - a condition in which they are denied 'prisoner of war' status while at the same time are not allowed to enjoy the rights recognized to criminal suspects under U.S. law," the group said.
The United States sent four Guantanamo prisoners home to Afghanistan and Pakistan on Monday, saying they could provide no useful intelligence information.
They were the first to be freed since the United States began sending suspected al Qaeda and Taliban prisoners to Guantanamo in January, except for one mentally ill prisoner repatriated to Afghanistan in April.
"All other detainees should be released without delay unless they are also to be tried in accordance with international human rights standards," Amnesty said.
About 30 more prisoners were flown to Guantanamo this week, putting the detainee population at 625, U.S. officials said.
Most were captured in Afghanistan during the U.S.-led military action against the al Qaeda group blamed for the Sept. 11 attacks on the United States.
The U.S. government, which came under strong criticism from Amnesty and other rights organizations when it began shipping the detainees to the camp at Guantanamo, does not recognize them as prisoners of war but says they are being treated humanely.
No charges have been filed against any of them, though President George W. Bush signed an order in November authorizing military tribunals to try them on terrorism charges.
Amnesty urged that any of the prisoners facing trial be granted access to lawyers and given fair trials.
It said the proposed tribunals would not meet international standards for fair trials because they lack independence from the executive branch and because they would not include the right to appeal to an independent and impartial court.