The next attack was the Warsaw-Poznań Operation, a part of the Vistula-Oder Offensive. On 13 January, 1BF began an offensive toward Pillkallen (Schlossberg between 1938 and 1945) in East Prussia, against which they met stiff resistance from the 3rd Panzer Army. The 1st Belorussian Front opened its attack on the German Ninth Army from the Magnuszew and Puławy bridgeheads at 08:30 on 14 January, again commencing with a heavy bombardment. The 33rd and 69th Armies broke out of the Puławy bridgehead to a depth of 30 km, while the 5th Shock and 8th Guards Armies broke out of the Magnuszew bridgehead. The 2nd and 1st Guards Tank Armies were committed after them to exploit the breach. On 25 January, the Front cut off the fortress city of Poznań which held 66,000 Germans, and continued its 80 km a day advance, leaving the 8th Guards Army to lay siege to the city, which they finally took on 23 February.