Secretary of Defense Lloyd Austin is visiting U.S. troops in Poland.
This comes as spiking tensions in eastern Ukraine on Friday aggravated Western fears of a Russian invasion and a new war in Europe, with a humanitarian convoy hit by shelling and pro-Russian rebels evacuating civilians from the conflict zone. A strong explosion hit the eastern city of Donetsk.
The Kremlin declared massive nuclear drills to flex its military muscle, and President Vladimir Putin pledged to protect Russia's national interests against what it sees as encroaching Western threats. U.S. and European leaders, meanwhile, grasped for ways to keep the peace and Europe's post-Cold War security order.
While Putin held out the possibility of diplomacy, a cascade of developments this week have have further exacerbated East-West tensions and fueled war worries.
This week's actions have fed those concerns: U.S. and European officials, focused on an estimated 150,000 Russian troops posted around Ukraine's borders, warn the long-simmering separatist conflict in eastern Ukraine could provide the spark for a broader attack.
Additional reporting by The Associated Press.