Frustrated in Moravia, the Prussians fortified Saxony and Silesia, while Frederick led an army northward to repel the advancing Russians, who had by then reached the borders of Brandenburg, where they besieged and burned Küstrin. The Prussian troops who had besieged Stralsund through the winter now withdrew to bolster Frederick's force, joining them near the ruins of Küstrin on 22 August. On 25 August a Prussian army of 35,000 men under Frederick engaged a Russian army of 43,000 under Fermor just east of the Oder in Neumark at the Battle of Zorndorf. Both sides fought to exhaustion and suffered heavy casualties, but the Russians withdrew, and Frederick claimed victory.