The Molotov-Ribbentrop Pact was the non-aggression pact between the Soviet Union and Nazi Germany prior to, and during WWII. It seems that the actions of the two countries after the invasion of Poland would confirm suspicions that they were working in concert in some fashion, but that doesn't mean that the specific secret agreements were known to the Allied powers.

My question is did Churchill, Roosevelt, or their administrations, know at any point prior to entering into an alliance with the Soviet Union that there were explicit agreements between Nazi Germany and the Soviet Union?