WASHINGTON (Reuters) - Russia, frustrated by early battlefield setbacks in Ukraine, could be shifting its strategy to siege warfare just as President Vladimir Putin raises the risk of a catastrophic miscalculation by putting nuclear forces on heightened alert, a senior U.S. defense official said on Sunday.
Putin gave the order to his nuclear forces as Washington assesses that Russian troops have made limited progress in their four-day-old invasion due to stiff Ukrainian resistance and planning failures that have left some units without fuel or other supplies, U.S. officials said.
