The Snowden leaks are "allowing them to burrow in and it's made it much more difficult for us to find them and the threats that they pose," Brennan added.
"This has caused grave damage to our national security," agreed DIA Director Army Lt. Gen. Michael Flynn. "The cost [is] to our nation in treasure, in capabilities that are going to have to be examined, reexamined and potentially adjusted. But I think that potentially the greatest cost is unknown today, but we will likely face, is the cost in human lives in tomorrow's battlefield in someplace where we put our military forces in harm's way."
Flynn was asked by Sens. Susan Collin, R-Maine, and Marco Rubio, R-Florida, if American troops could get killed because of Snowden, a computer whiz who once worked for the CIA and NSA contractors Dell and Booz Allen Hamilton.
The general said troops may die because of Snowden, who once worked in support of the military and even briefly enlisted in the Army before being quickly discharged.
"What we've seen the last six to eight months is an awareness by these [terrorist] groups...of our ability to monitor communications and specific instances where they've changed the ways in which they communicate to avoid being surveilled or being subject to our surveillance tactics," said NCTC Director Matthew Olsen.
However, when asked if Snowden's Russian hosts had accessed his unpublished NSA files despite the fugitive contractor's repeated denials, Clapper demurred.
"I think this might be best left to a classified session and I don't want to do any -- say or do anything that would jeopardize a current investigation," the intelligence chief told the Senate panel.
Little testimony about international espionage and terrorist threats was new or surprising, but officials reiterated warnings that the proximity of violent Islamist insurgents a few hundred miles from the Winter Olympic Games opening in Sochi, Russia on February 7 is worrisome.
"We are very focused on the problem of terrorism in the run-up to the Olympics," said Olsen, who recently visited the Sochi venues. He said the major threat for a terror attack is not at the game sites or venues, where Russia is already providing extensive security.
"The greater threat is to softer targets in the greater Sochi area and the outskirts beyond Sochi where there is substantial potential for a terrorist attack," he said.
FBI Director James Comey also insisted that cooperation with the Russian security agencies has been "steadily improving over the last year," even as other officials have said intelligence sharing should be significantly better.
The committee's chairman, Sen. Dianne Feinstein, D-California, also expressed her alarm over the gravitational pull of the Syrian civil war for hardened jihadis -- including U.S. persons -- who have flocked there easily to fight the pro-Iranian regime of Bashar al Assad.
Feinstein added that complacency about the al Qaeda threat has set in with the American public following the five-year pounding the group's leadership in Pakistan has sustained including the 2011 killing of Osama Bin Laden.
"I am concerned that this success has led to a popular misconception that the threat has diminished. It has not," Feinstein said.