In a question-and-answer session following a speech to a foreign affairs group in Baltimore on October 31, NSA Director General Keith Alexander was asked by a member of the audience what steps U.S. authorities were taking to stop Snowden from leaking additional information to journalists.
"I wish there was a way to prevent it. Snowden has shared somewhere between 50 (thousand) and 200,000 documents with reporters. These will continue to come out," Alexander said.
Alexander added that the documents were "being put out in a way that does the maximum damage to NSA and our nation," according to a transcript of his talk made available by NSA.
U.S. officials briefed on investigations into Snowden's activities have said privately for months that internal government assessments indicate that the number of classified documents to which Snowden got access as a systems operator at NSA installations ran into the hundreds of thousands.
Officials said that while investigators now believe they know the range of documents that Snowden accessed, they remain unsure which documents he downloaded for leaking to the media.
By comparison, the number of Pentagon and State Department documents leaked to WikiLeaks by a disgruntled U.S. Army private was much larger. The anti-secrecy group obtained around 400,000 Pentagon reports on the Iraq war, as well as 250,000 State Department cables and tens of thousands of documents on U.S. operations in Afghanistan.
None of the WikiLeaks material was classified higher than "Secret" but many NSA documents leaked by Snowden were marked "Top Secret" or with an even more restrictive "Special Intelligence" stamp. The material includes highly technical details on U.S. and allied eavesdropping activities.
Snowden's revelations, which first surfaced in June, are still causing a headache for the government of President Barack Obama, particularly in its dealing with allies.
For example, Germany was outraged by reports that the NSA monitored Chancellor Angela Merkel's cellphone.
Matthew Olsen, director of the National Counterterrorism Center in the Office of the Director of National Intelligence, said Snowden's leaks were "extremely damaging."
"There is no doubt that those disclosures have made our job harder. We've seen that terrorists or adversaries are seeking to learn about the ways that we collect intelligence and seeking to adapt and change the ways that they communicate," he told a congressional hearing on Thursday.
In the past few days, U.S. officials say, a panel of former officials and experts set up by Obama to review NSA operations in the wake of Snowden's disclosures has privately reported interim conclusions to the White House. The group's final report is due on December 15.
The report, along with the White House's own review, is likely to lead to policy changes to be announced by year's end. These are expected to include some constraints on the NSA's wide-ranging eavesdropping.
Also included in documents leaked by Snowden are at least 58,000 classified documents generated by Government Communications Headquarters, the NSA's British counterpart and eavesdropping partner, according to British authorities.Snowden is in Russia, where he was granted asylum in August for at least a year.