By this point Frederick was increasingly concerned by the Russian advance from the east and marched to counter it. Just east of the Oder in Brandenburg-Neumark, at the Battle of Zorndorf (now Sarbinowo, Poland), a Prussian army of 35,000 men under Frederick on 25 August 1758, fought a Russian army of 43,000 commanded by Count William Fermor. Both sides suffered heavy casualties—the Prussians 12,800, the Russians 18,000—but the Russians withdrew, and Frederick claimed victory.