WASHINGTON — 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.

While there is no evidence yet of any cable cutting, the concern is part of a growing wariness among senior American and allied military and intelligence officials over the accelerated activity by Russian armed forces around the globe. At the same time, the internal debate in Washington illustrates how the United States is increasingly viewing every Russian move through a lens of deep distrust, reminiscent of relations during the Cold War.

Inside the Pentagon and the nation’s spy agencies, the assessments of Russia’s growing naval activities are highly classified and not publicly discussed in detail. American officials are secretive about what they are doing both to monitor the activity and to find ways to recover quickly if cables are cut. But more than a dozen officials confirmed in broad terms that it had become the source of significant attention in the Pentagon.