We address the problem of clearing an arbitrary and unknown network of roads using an organized team of Unmanned Aerial Vehicles (UAVs) equipped with a monocular down-facing camera, an altimeter, plus high-bandwidth short- range and low-bandwidth long-range communication systems. We allow the UAVs to possibly split in several subgroups. In each subgroup a leader guides the motion employing a hierarchical coordination. A feature/image-based algorithm guides the subgroup toward the unexplored region without any use of global localization or environmental mapping. At the same time all the entry-points of the the explored region are kept under control, so that any moving object that enters or exits the previously cleared area. Simulative results on real aerial images demonstrate the functionalities and the effectiveness of the proposed algorithm.