Washington (CNN) -- If classified leaker Edward Snowden claims victory for disclosing details of U.S. surveillance programs, he should return all the other documents he has yet to to make public, Director of National Intelligence James Clapper said Wednesday.
Terrorists and other foes were "going to school" on information revealed by Snowden so far, Clapper told the Senate Intelligence Committee, adding the disclosures put from disclosures put U.S. intelligence operations and citizens at risk.
"What Snowden has stolen and exposed has gone way, way beyond his professed concerns with so-called domestic surveillance programs," Clapper said. "As a result, we've lost critical foreign intelligence collection sources, including some shared with us by valued partners."
Terrorists "going to school"
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, he continued.
"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."
A former National Security Agency contractor, Snowden 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 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.
Proposed surveillance changes
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.
Democratic Sen. Jay Rockefeller of West Virginia criticized Obama's idea of having third party organizations or phone companies keep the metadata records, rather than the NSA.
"The collection and querying of this metadata is not a private sector responsibility," Rockefeller said, adding that "going down this path will threaten, not strengthen, our ability to protect this country and the American people from a terrorist attack and massive invasions of their privacy."
Wednesday's committee hearing focused on the intelligence community's annual report of worldwide threats, and Clapper's opening statement outlined a series of crises and challenges amounted to a bleak outlook on the state of global affairs.
"Looking back over my more than half a century in intelligence, I have not experienced a time when we've been beset by more crises and threats around the globe," he said.
He provided an overview that read like a compilation of cheap thriller novels -- dispersed terrorist networks that led to last year's Boston Marathon bombing; Syria's civil war that he said created a "growing center of radical extremism"; that conflict's destabilizing impact on neighboring Lebanon, Iraq, Jordan and Turkey; increased foreign cyber capabilities; a "dangerous, unpredictable North Korea"; "perpetual conflict and extremism in Africa," and increased stress of growing populations that creates competition for energy, food and water.
Al Qaeda in 12 countries
Asked about al Qaeda, Clapper said it had spawned five different franchises operating in 12 countries including Yemen, Somalia, Syria and elsewhere.
He was especially concerned about Syria, saying it had become "a huge magnet for extremists" with an estimated 26,000 of them among 1,600 groups in the war-torn Middle East nation.
"We estimate, at this point, in excess of 7,000 foreign fighters have been attracted from some 50 countries, many of them in Europe and the Mideast," Clapper added.
The foreign extremists "engage in combat, get training, and we're seeing now the appearance of training complexes in Syria to train people to go back to their countries, and, of course, conduct more terrorist acts," he said, calling it a "huge concern."
North Korea nuclear program
Regarding the often unnerving actions of North Korea, Clapper said Kim Jong Un's regime appeared to have gone ahead with plans it announced last spring to step up activities at its nuclear facilities.
North Korea has restarted a plutonium production reactor and expanded a uranium enrichment plant at Yongbyon, its main nuclear complex, he said in his written testimony to the committee.
His assessment tallies with researchers' analysis of satellite images of the Yongbyon complex from August.
Clapper also warned that North Korea is "committed to developing a long-range missile technology that is capable of posing a direct threat to the United States."
But he said the longstanding view of U.S. intelligence is that Pyongyang sees its nuclear program as "intended for deterrence, international prestige and coercive diplomacy."
CNN's Jamie Crawford and Jethro Mullen contributed to this report.