The length of the German advance created chronic supply difficulties, particularly of petrol; the Black Sea was judged too dangerous and fuel was brought by rail through Rostov or delivered by air, but panzer divisions were sometimes at a standstill for weeks. Even petrol trucks ran out of fuel and oil had to be brought up on camels. With the Soviets often retreating instead of fighting, the number of prisoners fell short of expectations and only 83,000 were taken. As Hitler and OKH began to concentrate on Stalingrad, some of Kleist's mobile forces were diverted. Kleist lost his flak corps and most of the "Luftwaffe" supporting the southern front, only reconnaissance aircraft being left behind. The Voyenno-Vozdushnye Sily (VVS) brought in about 800 bombers, a third of which were operational. With the transfer of air cover and flak units, Soviet bombers were free to harass the German advance. The quality of the Soviet resistance increased, with many of the forces used coming from local levies, who Kleist thought were willing to fight harder for their homeland. German units were especially bogged down by fighting Georgian alpine and mountain troops, who greatly contributed to stalling their advance. The quantity of replacements and supplies the Soviets committed increased, and faced with these difficulties, the Axis advance slowed after 28 August.