The senior official, who asked not to be identified, gave no details, but told reporters travelling with US Defence Secretary Donald Rumsfeld to Iraq on Thursday that the ICRC had delivered the report to the US State Department this week.
"It was described to him (Rumsfeld) as critical," the official said, noting that the Red Cross had repeatedly complained about the imprisonment at Guantanamo of more than 500 people - most of them captured in Afghanistan - in stark cells for up to two years or more without charges.
ICRC spokesman Florian Westphal in Geneva confirmed the agency had handed over a confidential report on Guantanamo to US authorities as part of its standard procedure, but declined to give any details.
Earlier, Westphal said the ICRC was concerned the Guantanamo detainees were in legal limbo "beyond the reach of the law".
ICRC President Jakob Kellenberger raised the issue with top
US officials in January, saying in a statement at the time the agency's concerns had "not yet been adequately addressed".
"That remains entirely valid," Westphal said on Thursday. A recently publicised ICRC report on conditions at US prisons in Iraq fuelled international outrage over abuse of detainees by American military police. The ICRC found abuse was "in some cases tantamount to torture".
The organisation has condemned
conditions in Iraqi prisons
The New York Times, citing counterterrorism officials, reported on Thursday that CIA interrogation methods used to extract information from al-Qaida suspects at Guantanamo and other undisclosed locations are so severe the FBI has told its agents to stay away from the sessions.
It said FBI officials had advised the bureau's director, Robert Mueller III, that the interrogation techniques, which would be prohibited in criminal cases, could compromise their agents in future criminal cases.