Emboldened by the plunder from the Franks, the Avars resumed their raids across the Danube in autumn of 597, catching the Byzantines by surprise. The Avars even caught Priscus' army while it was still in its camp in Tomis, and laid siege to it. However, they lifted the siege on 30 March 598, at the approach of a Byzantine army led by Comentiolus, which had just crossed Mount Haemus and was marching along the Danube up to Zikidiba, only 30 kilometres (19 mi) from Tomis. For unknown reasons, Priscus did not join Comentiolus when he pursued the Avars. Comentiolus made camp at Iatrus, however he was routed by the Avars, and his troops had to fight their way back over the Haemus. The Avars took advantage of this victory and advanced to Drizipera, near Constantinople. At Drizipera the Avar forces were struck by a plague, leading to the death of a large portion of their army, and seven sons of Bayan, the Avar Khagan.