Washington (CNN) -- In the world of military strategy, every contingency must be examined, especially the worst-case scenario.
Lt. Gen. Michael Flynn, the head of the Defense Intelligence Agency, made that clear when he told National Public Radio in an interview broadcast Friday how U.S. officials must plan for the possibility that Vladimir Putin's Russia has access to American battle plans and other secrets possibly taken by classified leaker Edward Snowden.
"If I'm concerned about anything, I'm concerned about defense capabilities that he may have stolen from where he worked, and does that knowledge then get into the hands of our adversaries — in this case, of course, Russia," Flynn said of the former National Security Agency contractor who fled to Moscow to seek asylum.
A hero to some and traitor to others, Snowden last year disclosed details of the vast U.S. surveillance network put in place after the September 11, 2001, terrorist attacks, including how the government keeps records on billions of phone calls for possible use in terrorism investigations.
Flynn said he worried about what else Snowden knows, and how Russia -- where Snowden lives now -- may have access to the documents. He cited intelligence capabilities, operational capabilities, technology and weapons systems as potential subjects of so far unpublicized information Snowden -- and Russia -- may have.
"We really don't know" what Snowden's got, Flynn said, adding that "we have to assume the worst case and then begin to make some recommendations to our leadership about how do we mitigate some of the risks that may come from what may have been compromised."
He added that the intelligence community also must assume that Russia either already has the information taken by Snowden or is trying to get it, adding "that would be very serious."
Because of the possibility, "we have to make some judgments, recommendations about ... how to respond to that," Flynn said.
"We're going to be dealing with this for many, many years," he noted, saying procedures, techniques and tactics currently in use may have to be changed.
Flynn spoke as Putin has moved troops into the Crimea Peninsula of Ukraine in a showdown with the United States and its European allies over the former Soviet region's independence.
In January, he and Director of National Intelligence James Clapper told the Senate Intelligence Committee that the Snowden leaks already caused serious damage to U.S. security.
"What Snowden has stolen and exposed has gone way, way beyond his professed concerns with so-called domestic surveillance programs," Clapper said then. "As a result, we've lost critical foreign intelligence collection sources, including some shared with us by valued partners."
Terrorists and other adversaries of America were "going to school on U.S. intelligence sources' methods and trade craft, and the insights that they are gaining are making our job much, much harder," Clapper told the committee.
"Snowden claims that he's won and that his mission is accomplished," Clapper also noted. "If that is so, I call on him and his accomplices to facilitate the return of the remaining stolen documents that have not yet been exposed to prevent even more damage to U.S. security."
Flynn told the panel that "the greatest cost that is unknown today but that we will likely face is the cost of human lives on tomorrow's battlefield or in some place where we will put our military forces when we ask them to go into harm's way."
CNN Pentagon Correspondent Barbara Starr contributed to this report.