Jakob Kellenberger, president of the International Committee of the Red Cross (ICRC), made the announcement at a 9 December news conference in Geneva.
He said the ICRC is engaged in an "intense dialogue" with the United States in a bid to get access to secretly held terror suspects.
His comments come one day after a senior U.S. State Department official acknowledged to reporters in Geneva that the ICRC does not have access to all detainees held by U.S. forces.
Human rights groups accuse the CIA of running secret prisons and covertly transporting detainees. They say secret detentions are against international law, and that the practice often leads to torture.