The two fronts spent December training and preparing for the offensive, and received significant reinforcements. These included not just replenishment and additional rifle divisions and brigades, but also significant additional artillery and engineer units, which were vital for breaching the heavy German defenses. Specialized winter units included three ski brigades and four "aerosani" battalions. To ensure the Soviet forces had air superiority, which they had lacked in the previous offensive, the air strength in the area was increased to a total of over 800 planes, predominantly fighters. Large tank forces could not operate well in the swampy terrain, so the tank forces were used primarily as battalions reinforcing divisions or slightly larger brigades, which were to operate independently.