The US State Department announced Sunday it was ordering diplomats' family members to leave Ukraine, as US President Joe Biden weighed options for boosting America's military assets in Eastern Europe to counter a buildup of Russian troops.

The order, which also allowed US diplomats stationed at the embassy in the Ukrainian capital Kyiv to leave voluntarily, was one of the clearest signs yet that American officials are bracing for an aggressive Russian move in the region.

"Military action by Russia could come...