In spite of the heaviest kind of bombardment and of almost uninterrupted infantry attacks on Kovno and Novo Georgievsk, both of these fortresses still held out. By August 1, 1915, however, the German lines had advanced far beyond these places and it became clear that their next chief objective was Brest-Litovsk. Each one of the three main army groups directed strong parts of their forces toward this Russian stronghold. From the northwest detachments of Von Hindenburg's group, coming from Lomza and Ostroff, had crossed in a wide front the Warsaw-Bialystok section of the Warsaw-Vilna-Petrograd railway. After taking Briansk they had forced the crossing of the Nurzets, a tributary of the Bug, and the only natural barrier in front of Brest-Litovsk from that direction. They were rapidly approaching the Brest-Litovsk-Bialystok railway. The central group's front--Lukoff-Siedlets-Sokoloff--had been pushed forward to Drohichin on the Bug, only about forty-five miles to the northeast of the fortress. Parts of Von Mackensen's southern group under the Archduke Joseph Ferdinand had even reached Biala, less than twenty miles west of Brest-Litovsk, and still other detachments from this group were advancing along the eastern bank of the Bug. Three railroads leading out of the fortress were still in the hands of the Russians--to Bialystok to the north, to Pinsk and Minsk to the east, and to Kovel and Kovno to the south. This continuous offensive against all the Russian lines, of course, cost both sides dearly. The attackers, however, seemed to have had the better end of it. The Russians, according to official figures, lost almost 100,000 men by capture alone during the first two weeks of August, 1915. The German successes before Kovno and Novo Georgievsk had the result of increasing the vigor of the drive against Brest-Litovsk. Those detachments of Von Hindenburg's army group which had forced a crossing of the Nareff between Bialystok and Lomza pushed on rapidly to the south and threatened as early as August 18, 1915, the northern section of the Bialystok-Brest-Litovsk railway. On the same day Prince Leopold's forces reached the south bank of the Bug, north of Sarnaki. Parts of Von Mackensen's army kept up its attack against the Russians around Biala, forced them across the Bug and into the very forts of Brest-Litovsk and at the same time began the bombardment of the outlying forts with the heavy artillery that had been brought up. Other parts, on that day, August 19, 1915, crossed the northern part of the Cholm-Brest-Litovsk railway east of Vlodava. At the same time Austrian forces under Field Marshal-Lieutenant von Arz and Archduke Joseph Ferdinand cleared the left bank of the Bug, east of Janoff, and thereby completed the investment of the fortress from the west.