Passage: On 16 December, the Soviets launched Operation Little Saturn, which attempted to punch through the Axis army (mainly Italians) on the Don and take Rostov. The Germans set up a "mobile defense" of small units that were to hold towns until supporting armor arrived. From the Soviet bridgehead at Mamon, 15 divisions – supported by at least 100 tanks – attacked the Italian Cosseria and Ravenna Divisions, and although outnumbered 9 to 1, the Italians initially fought well, with the Germans praising the quality of the Italian defenders, but on 19 December, with the Italian lines disintegrating, ARMIR headquarters ordered the battered divisions to withdraw to new lines.The fighting forced a total revaluation of the German situation. Sensing that this was the last chance for a breakout, Manstein pleaded with Hitler on Dec 18th, but Hitler refused. Paulus himself also doubted the feasibility of such a breakout. The attempt to break through to Stalingrad was abandoned and Army Group A was ordered to pull back from the Caucasus. The 6th Army now was beyond all hope of German relief. While a motorised breakout might have been possible in the first few weeks, the 6th Army now had insufficient fuel and the German soldiers would have faced great difficulty breaking through the Soviet lines on foot in harsh winter conditions. But in its defensive position on the Volga, the 6th Army continued to tie down a significant number of Soviet Armies.On 23 December, the attempt to relieve Stalingrad was abandoned and Manstein's forces switched over to the defensive to deal with new Soviet offensives. As Zhukov states, "The military and political leadership of Nazi Germany sought not to relieve them, but to get them to fight on for as long possible so as to tie up the Soviet forces. The aim was to win as much time as possible to withdraw forces from the Caucasus (Army Group A) and to rush troops from other Fronts to form a new front that would be able in some measure to check our counter-offensive."

Question: Besides the weather condistions, what was stopping the 6th army?
insufficient fuel