While the main operations were directed eastwards to central and northern Germany, elements of three U.S. Armies concentrated on the pocket, taking it section by section. Model's troops put up a strong resistance along the Dortmund–Ems Canal and the Sieg river-line, holding their ground from 4 April to 9 April and launching a counterattack against U.S. 75th and 95th Divisions near Dortmund. For every German city or town that capitulated, another fought on for every building. Bürgermeisters of some German cities presented white flags to the invading U.S. troops, such as at Duisburg and Essen while German troops at Dortmund, Wuppertal and Hamm fought fanatically for days to the complete exhaustion of all available potential. The presence of SS troops was a common element in most instances of all-out resistance.