WASHINGTON (CNN) -- Director of National Intelligence James Clapper on Wednesday asked classified leaker Edward Snowden and his accomplices to turn over any intelligence documents they have yet to make public, warning that terrorists and other foes were "going to school" on information from disclosures so far.
Clapper spoke at a Senate hearing on the annual report of worldwide threats, and his opening statement outlined a series of crises and challenges around the world that he called the most significant he has ever experienced.
He said Snowden's disclosures have put U.S. intelligence operations and citizens at risk.
Snowden, a former National Security Agency contractor, is in Russia seeking permanent asylum to avoid U.S. criminal charges over the leaking of classified documents that exposed surveillance programs, including the collection of phone records for possible use in terrorism investigations.
Asked about the impact of the Snowden leaks at the same hearing, Defense Intelligence Agency Director Lt. Gen. Michael Flynn told the panel the disclosures have caused "grave damage" to U.S. national security.
"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," Flynn said.
President Barack Obama has proposed modest reforms to the surveillance programs disclosed by Snowden, but it remains unclear if a divided Congress will come to any agreement on changes intended to balance privacy concerns with national security needs.
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