ICRC officials, after a visit to the military base, said many detainees were suffering "a worrying deterioration" in mental health because they are held without charges and without legal counsel.
"They have no idea about their fate and they have no means of recourse at their disposal through any legal mechanism," said ICRC spokesman Florian Westphal.
Some detainees, suspected of links to the fallen Afghan Taliban regime or al Qaeda terrorist network, have been at Guantanamo for more than 18 months.
Westphal said the ICRC, the only independent body with access to the detainees, had yet to see "any significant movement" from U.S. officials to its long-standing request that the United States give the detainees legal rights in accordance with international conventions governing prisoners or war.
"We have observed what we consider to be a worrying deterioration in the psychological health of a large number of the internees" because of the uncertainty of their situation, Westphal told The Associated Press.
International rights groups say the indefinite detentions without charge, which have led to 32 suicide attempts by 27 detainees, are inhumane.
An AP reporter in Guantanamo had arranged to meet with the ICRC at the base on Wednesday, but U.S. military officials refused to allow the interview, saying permission had not been granted by the Department of Defense.
The ICRC team met with U.S. officials Thursday in Guantanamo, winding up its two-month visit to the camp.
The neutral, Swiss-run organization has been appealing in private to the Bush administration for due process since soon after the detention center was opened in early 2002, the spokesman noted.
In an unusual move, ICRC went public with that appeal in May its president, Jakob Kellenberger, met with top officials of the Bush administration in Washington.
A statement posted on the ICRC Web site in August notes its ongoing concern "about the impact the seemingly open-ended detention is having on the internees."
"As the internees spend more time in Guantanamo and continue to have no idea what is going to happen to them, we are concerned that the impact on them will get more serious," Westphal said.
Already, he added, "what we are seeing during our repeated visits is that this uncertainty has definitely had an impact on the internees."
During the current visit, Red Cross representatives carried out private interviews with many of the detainees, he said. Those held at Guantanamo come from more than 40 countries and speak around 17 languages, the ICRC said.
Red Cross teams also have been giving the detainees messages from their families and have collected their return messages — so far about 8,000. For many, the letters are their main way of staying in touch with relatives.
The agency, which is the guardian of the Geneva Conventions on the conduct of warfare, noted that the U.S. has refused to grant the detainees prisoner of war status but promised to treat them humanely.
Whatever the status, the ICRC said: "People held as a result of conflict or armed violence are protected by international humanitarian law, and should be treated humanely."