The Allies were using Norwegian airfields to transfer aircraft to the Finns fighting against the Soviets in the Winter War, as well as mining Norwegian waters and the Germans were alarmed by these developments. If the allies were to use Norwegian naval bases or successfully mine Norwegian waters, they could cut off the vital iron ore imports from Sweden and tighten the blockade of Germany. The Allies also had made plans to invade Norway and Sweden in order to cut off the iron ore shipments to Germany. Admiral Rolf Carls, commander of the Kriegsmarine in the Baltic sea region, proposed the invasion of Norway to Raeder in September 1939. Raeder briefed Hitler on the idea in October, but planning did not begin until December 1939. The operation was in low-priority planning until the Altmark Incident, but found a new sense of urgency thereafter. The invasion proved costly for the Kriegsmarine, losing a heavy cruiser, two of its six light cruisers, 10 of its 20 destroyers and six U-boats. In addition, almost all of the other capital ships were damaged and required dockyard repairs, and for a time the German surface fleet had only three light cruisers and four destroyers operational in the aftermath of the Norwegian Campaign.