Russia announced massive nuclear drills while Western leaders grasped Friday for ways to avert a new war in Europe amid soaring East-West tensions, after unusually dire U.S. warnings that Moscow could order an invasion of Ukraine any day.

Immediate worries focused on the volatile front lines of eastern Ukraine, where an upsurge of recent shelling tore through the walls of a kindergarten and basic communication was disrupted. Western officials, focused on an estimated 150,000 Russian troops posted around Ukraine’s borders, fear the long-simmering conflict could provide the spark for a broader war.