For its final attack on Leningrad, the 4th Panzer Group was reinforced by tanks from Army Group Center. On 8 August, the Panzers broke through the Soviet defences. By the end of August, 4th Panzer Group had penetrated to within of Leningrad. The Finns had pushed southeast on both sides of Lake Ladoga to reach the old Finnish-Soviet frontier. The Germans attacked Leningrad in August 1941; in the following three "black months" of 1941, 400,000 residents of the city worked to build the city's fortifications as fighting continued, while 160,000 others joined the ranks of the Red Army. Nowhere was the Soviet "levée en masse" spirit stronger in resisting the Germans than at Leningrad where reserve troops and freshly improvised "Narodnoe Opolcheniye" units, consisting of worker battalions and even schoolboy formations, joined in digging trenches as they prepared to defend the city. On 7 September, the German 20th Motorized Division seized Shlisselburg, cutting off all land routes to Leningrad. The Germans severed the railroads to Moscow and captured the railroad to Murmansk with Finnish assistance to inaugurate the start of a siege that would last for over two years. At this stage, Hitler ordered the final destruction of Leningrad with no prisoners taken, and on 9 September, Army Group North began the final push. Within ten days it had advanced within of the city. However, the push over the last proved very slow and casualties mounted. Hitler, now out of patience, ordered that Leningrad should not be stormed, but rather starved into submission. Along these lines, the OKH issued Directive No. la 1601/41 on 22 September 1941, which accorded Hitler's plans. Deprived of its Panzer forces, Army Group Center remained static and was subjected to numerous Soviet counterattacks, in particular the Yelnya Offensive, in which the Germans suffered their first major tactical defeat since their invasion began; this Red Army victory also provided an important boost to Soviet morale. These attacks prompted Hitler to concentrate his attention back to Army Group Center and its drive on Moscow. The Germans ordered the 3rd and 4th Panzer Armies to break off their Siege of Leningrad and support Army Group Center in its attack on Moscow. Before an attack on Moscow could begin, operations in Kiev needed to be finished. Half of Army Group Center had swung to the south in the back of the Kiev position, while Army Group South moved to the north from its Dnieper bridgehead.