Based on a visit to the prison in June, an ICRC team that included humanitarian workers and experienced medical personnel found a system devised to break the will of prisoners through "humiliating acts, solitary confinement, temperature extremes, use of forced positions", the New York Times reported on Tuesday.
"The construction of such a system, whose stated purpose is the production of intelligence, cannot be considered other than an intentional system of cruel, unusual and degrading treatment and a form of torture," the report said.
Sent to the US government in July, the report - a detailed memorandum of which was obtained recently by the New York Times - said the prisoners were also exposed to loud and persistent noise and music as well as to prolonged cold.
It also said that detainees were subjected to "some beatings", the daily added.
The daily said the report did not mention how many of the 550 detainees currently being held at Guantanamo were subjected to such treatment.
The United States has used the military facility to hold people detained around the world in its "war on terror" indefinitely and without trial.
The ICRC team also found that medical personnel at the Guantanamo prison provided information about the prisoners' mental health and vulnerabilities to interrogators, a practice a human rights expert told the daily was in violation of international ethical standards.
Physicians allegedly provide info
about vulnerable prisoners
The daily said a statement from the Pentagon said "the allegation that detainee medical files were used to harm detainees is false".
"The United States operates a safe, humane and professional detention operation at Guantanamo that is providing valuable information in the war on terrorism," the Pentagon statement said.
No legal access
Personnel assigned to Guantanamo, the Pentagon statement added, "go through extensive professional and sensitivity training to ensure they understand the procedures for protecting the rights and dignity of detainees".
Transferred from military prisons in Iraq and Afghanistan, many of the detainees at Guantanamo have been there for almost three years without any access to lawyers.
Only four have been charged, as the US government resists a legal onslaught over its handling of al-Qaida and Taliban detainees, most of whom it believes fall outside the protection of the Geneva Conventions.