A top Red Cross official has broken with tradition by publicly attacking conditions at the US military base on Cuba where al-Qaeda suspects are being held.
Guantanamo security is especially tight after recent spy fears
Christophe Girod - the senior Red Cross official in Washington - said it was unacceptable that the 600 detainees should be held indefinitely at Guantanamo Bay without legal safeguards.
The Red Cross is the only organisation with access to the detainees.
His criticism came as a group of American former judges, diplomats and military officers called on the US Supreme Court to examine the legality of holding the foreign nationals for almost two years, without trial, charge or access to lawyers.
Mr Girod said the International Committee of the Red Cross (ICRC) was making the unusually blunt public statement because of a lack of action after previous private contacts with American officials.
"One cannot keep these detainees in this pattern, this situation, indefinitely," he said during a visit to the US naval base where the Taleban and al-Qaeda suspects are being held.
US officials insist there are reasons for holding the alleged fighters and say they will get a fair legal hearing in due course.
Mr Girod is leading a team from the ICRC which has just completed an inspection tour of the detention camp in Cuba.
Although he did not criticise any physical conditions at the camp, he said that it was intolerable that the complex was used as "an investigation centre, not a detention centre".
"The open-endedness of the situation and its impact on the mental health of the population has become a major problem," he told the New York Times.
Christine Huskey, an American lawyer representing 28 Kuwaiti inmates, told the BBC she had had "absolutely" no access to them.
"I represent a ghost," she said.
In the past 18 months, 21 detainees have made 32 suicide attempts, and many more are being treated for depression, the New York Times says.
Mr Girod says prisoners who spoke to his team regularly asked about what was going to happen to them.
"It's always the number one question," he said. "They don't know about the future."
United States Navy base in south-eastern Cuba
Leased by Washington since 1903, but not regarded as US territory
Houses more than 600 al-Qaeda and Taleban suspects
Inmates not covered by US constitutional guarantees
Camp officials have said most of the detainees' mental health problems existed before they arrived.
The Geneva-based ICRC is the only group outside the US Government allowed to visit the detention camp.
In exchange for access, the committee has agreed to take any initial complaints directly to Washington. It publicises its views only when it feels they are not being heeded.
In this instance, the ICRC says it has been urging the White House for months to make significant changes in Guantanamo.
The administration, Mr Girod said, should consider establishing a policy of giving detainees some idea of when they can learn whether they will be charged or released.
On Sunday a group including former American judges and military officials filed legal papers urging the US Supreme Court to intervene.
Don Guter, the US navy's judge advocate general until last year, said it was not acceptable simply to hold suspected al-Qaeda or Taleban members until the US war on terror was over.
US legal experts have begun to press for change
The argument filed to the Supreme Court by Mr Guter and others said: "The lives of American military forces may well be endangered by the United States' failure to grant foreign prisoners in its custody the same rights that the United States insists be accorded to American prisoners held by foreigners."
That view was backed by ex-prisoners-of-war, some of whom told the Supreme Court they owed their lives to the fact that their captors abided by the Geneva conventions.
On Wednesday an Australian lawyer representing some of the suspects said they were being submitted to torture.
US officials have denied torturing detainees, saying they are allowed to practise their religion and given good medical care.