The London-base human-rights group, Amnesty International, has issued a new report condemning what it calls the "unlawful detentions" of about 500 men at the American naval base at Guantanamo Bay, Cuba.
The 11-page report gives details of an ongoing hunger strike and a series of suicide attempts at the Guantanamo detention center.
One of the report's authors, Sharon Critoph, says the situation has been a concern for Amnesty International for the past four years.
"We've been calling for all those detained to either be charged and tried in accordance with international law or to be released," said Critoph. "It's a very unfortunate anniversary now to think that it's now over four years since we first made that call - four years since the first men were transferred to this detention center."
Critoph says Amnesty International is concerned that the United States may decide to hand over Guantanamo prisoners to their home countries, where they could be held indefinitely.
"Guantanamo has now become an embarrassment to the U.S. authorities and we know they are trying to secure arrangements with a number of different countries about the mass transfers," she added. "What we do not want to see is for these people now to be sent to other countries where they may face torture and ill treatment beyond the scrutiny of the world."
The United States first opened the Guantanamo detention center to hold what are called "enemy combatants" captured during the invasion of Afghanistan in 2001.
U.S. officials have described the Guantanamo prisoners at the "worst of the worst," but strongly deny allegations that the detainees are mistreated.
President Bush has criticized previous Amnesty International reports on Guantanamo as "absurd" and based on interviews with detainees who hate America and who are trained to lie.