Although the Bulgarian government had officially joined the Axis Powers, it maintained a course of military passivity during the initial stages of the Invasion of Yugoslavia and the Battle of Greece. German, Italian, and Hungarian troops crushed opposing forces of Yugoslavia and Greece but on 6 April 1941, Yugoslavian airplanes bombed the Bulgarian town of Kyustendil, with 67 people killed and 90 wounded and a suburb of Sofia where 8 people were killed. The Yugoslav government surrendered on April 17. The Greek government was to hold out until April 30. On 18 April 1941 the Bulgarian government received a telegram from Joachim von Ribbentrop in which he specified the regions to be taken by units of the Bulgarian army. In Greece, the units were to occupy Thrace, as well as Macedonia between the Strymon and Nestos rivers. In Yugoslavia, the Bulgarians were to occupy an area from the river Vardar and Pomoravlje to the Pirot-Vranje-Skopje line. Ribbentrop's telegram said that the line was temporary, i.e., that it could be moved to the west of the river Vardar as well.