By Mark Hosenball, Matt Spetalnick and Peter Apps
WASHINGTON (Reuters) - Even as U.S. intelligence agencies and their global partners assess potential damage from Edward Snowden's disclosures about surveillance programs, militants have begun responding by altering methods of communication, a change that could make it harder to foil attacks, U.S. officials say.
Intelligence agencies have detected that members of targeted militant organizations, including both Sunni and Shi'ite Islamist groups, have begun altering communications patterns in what was believed to be a direct response to details on eavesdropping leaked by the former U.S. spy agency contractor, two U.S. national security sources said.
The officials said it was too early to tell whether the recent changes in communications methods had caused a loss of critical intelligence or if there was now a greater risk of missing warning signs about future attacks. "You don't know what you lose until after you've lost it," one of the sources said.
Previous dire warnings of leaks causing huge damage to U.S. national security interests have proved overplayed. The leaking of tens of thousands of U.S. diplomatic cables by Wikileaks in 2010 appears to have had far less impact than Washington initially warned.
And militants changing the way they communicate could actually hinder plots as they resort to different methods to avoid detection.
The charge that Snowden's leaks are causing damage, made by officials speaking on condition of anonymity, comes as the Obama administration mounts a campaign to pressure Russia to extradite him. Russian president Vladimir Putin on Tuesday confirmed Snowden was in the transit area of a Moscow airport, but ruled out handing him to Washington.
The officials declined to specify what changes were spotted among militant groups, fearing that the more details provided on what was known about their behavior the easier it would be for them to adapt.
One U.S. telecommunications expert said privately that the militants' latest adjustments likely included reduced electronic transmissions and more frequent switching of cell phones while they seek new encryption methods.
Flashpoint Global Partners, which monitors Islamist militant websites, has noted increased discussion of the NSA leaks.
"The simple rule is that 'don't be a fool' don't give ur real information on internet through email u use for jihadi activities. just use fake names and TOR browser is must," one person posted, referring to an Internet browser that can help the user remain anonymous and avoid surveillance.
Exactly how serious the damage might be is difficult to measure as intelligence agencies do not know how much more sensitive material Snowden possesses that has not yet been published.
Militants have a long history of trying to cover their electronic tracks.
Largely using written messages and trusted couriers, al Qaeda leader Osama bin Laden hid for years until he was killed by U.S. forces in 2011. He advised militants to exercise extreme caution over electronic communications. Thumb drives and sim cards used to carry information should be destroyed after use, he told them.
Intelligence analysts around the world are working to determine the operational impact of Snowden's exposure of methods used by the U.S. National Security Agency and Britain's Government Communications Headquarters agency to tap into telephone and Internet traffic.
Among the potential damage to Western intelligence is that foreign governments like Russia and China may have found out technical details of how Western agencies intercept communications and what specific links they monitor or tap into.
U.S. officials fear that if Snowden, now allied with the anti-secrecy organization WikiLeaks, releases more of the large cache of classified information he is believed to possess, there could be further fallout for Western counterterrorism operations.
U.S. agencies are already operating on a "worst case" assumption that all of the classified material in Snowden's possession has somehow made its way to one or more adversary intelligence services, several sources said.
How much of a long-term impact Snowden's actions have at home may depend largely on whether a backlash in the United States ultimately restricts how the NSA can use its tools.
"The problem is one of public trust, a commodity now in very short supply," a former Western intelligence official said.
U.S. and British officials insist that massive telephone and Internet surveillance by the NSA and GCHQ, including collection of raw data about U.S. phone calls and tapping by GCHQ into fiber optic trunk cables carrying Internet traffic, has stopped attacks and is essential to public security.
The Office of the National Counterintelligence Executive, is still in the early stages of damage assessment from the Snowden leaks and it is not yet known when it will issue its findings, a U.S. intelligence official said.
With Snowden's disclosures pointing to U.S. electronic surveillance of Chinese targets, Washington may face charges of a double standard when it holds cyber-security talks with Beijing next month.
"How can the United States continue to stand on high moral ground after demonizing China and accusing us of hacking?" a source close to the Chinese leadership told Reuters in Beijing.
That stance is seen as a logical result of the Snowden saga by some American experts too.
"It will make it more difficult for us to appear holier than thou," said James Lewis, a former State Department official who is now a senior fellow at the Center for Strategic and International Studies in Washington.
(Additional reporting by Tabassum Zakaria in Washington and Benjamin Kang Lim and Michael Martina in Beijing; editing by Jackie Frank)