WASHINGTON -- Suspected terrorists have changed how they communicate and have become more difficult to track as a result of former contractor Edward Snowden's disclosures about U.S. surveillance operations, according to current and former officials who say the changes have led to a significant loss of intelligence.
How much that loss amounts to remains unknown as the government's classified assessment is continuing, they said.
In addition, Mr. Snowden's disclosures about eavesdropping in Russia and China gave each of those nations insights that are already thought to have impaired the National Security Agency's ability to intercept their communications, the officials said.
Among disclosures from Mr. Snowden that were published in The Washington Post and Britain's The Guardian was that Skype, the Internet calling service, was among the systems that provided data to the NSA's secret PRISM database. That disclosure contradicted a widespread belief that Skype calls were difficult or impossible to intercept.
Some alleged terrorists the NSA was tracking are no longer using Skype, U.S. officials say. Others have stopped using email, said one U.S. official briefed on the damage. "The Skype thing was really bad," the official said.
The inability to use such common communications systems creates problems for terrorist groups by reducing their ability to share plans and coordinate, but it also costs intelligence agencies information, the official said.
On the foreign intelligence side, government officials are still trying to determine how much specific information Mr. Snowden knew about the methods the United States uses to eavesdrop on other nations. Former counterintelligence officials say they believe that there has already been serious damage, and fear that it could worsen, depending on what Mr. Snowden had.
Mr. Snowden's father on Friday told NBC News that he believes that his son would return to the United States if he were assured that he would not be jailed before trial or subjected to a gag order.
Lonnie Snowden told journalist Michael Isikoff that he has not spoken since April with his son, who is now believed to be hiding in a Moscow airport to evade arrest by U.S. authorities. He has been charged with leaking classified documents.
Mr. Snowden, a career Coast Guard officer who retired and moved a few years ago to Upper Macungie Township, Pa., an Allentown suburb in Lehigh County, told NBC that he did not trust WikiLeaks, the anti-secrecy group providing his son with legal and logistical assistance.
"I am concerned about those who surround him," the father said in the interview. "I think WikiLeaks, if you've looked at past history, you know, their focus isn't necessarily the Constitution of the United States. It's simply to release as much information as possible."
Washington Post contributed.