Russian submarines and spy ships are aggressively operating near the vital undersea cables that carry almost all global Internet communications, raising concerns among some American military and intelligence officials that the Russians might be planning to attack those lines in times of tension or conflict.

The issue goes beyond old worries during the Cold War that the Russians would tap into the cables — a task American intelligence agencies also mastered decades ago. The alarm today is deeper: The ultimate Russian hack on the United States could involve severing the fiber-optic cables at some of their hardest-to-access locations to halt the instant communications on which the West's governments, economies and citizens have grown dependent.