Information regarding mass murders of Jews began to reach the free world soon after these actions began in the Soviet Union in late June 1941, and the volume of such reports increased with time. The early sources of information include German police reports intercepted by British intelligence; local eyewitnesses and escaped Jews reporting to the underground, Soviet, or neutral sources; and Hungarian soldiers on home leave, whose observations were reported by neutral sources. During 1942, reports of a Nazi plan to murder all the Jews – including details on methods, numbers, and locations – reached Allied and neutral leaders from many sources, such as the underground Jewish Socialist Bund party in the Warsaw ghetto in May; Gerhard Riegner's cable from Switzerland in August; the eyewitness account of Polish underground courier Jan Karski in Novemer; and the eyewitness accounts of 69 Polish Jews who reached Palestine in a civilian prisoner exchange between Germany and Britain in November.
On December 17, 1942, the Allies issued a proclamation condemning the
extermination of the Jewish people in Europe and declared that they would punish the perpetrators. Notwithstanding this, it remains unclear to what extent Allied and neutral leaders understood the full import of their information. The utter shock of senior Allied commanders who liberated camps at the end of the war may indicate that this understanding was not complete.
Source: Yad Vashem