Doug Kasputin / Reuters file National Security Agency Director Gen. Keith Alexander speaks in Baltimore on June 27.

The U.S. Army general who runs the National Security Agency, in charge of the government surveillance programs revealed by Edward Snowden, say the disclosures and the resulting fallout have greatly distorted how they actually work and have caused some intelligence sources to dry up.

"We have concrete proof that they have already, terrorists groups and others, are taking action, making changes, and it's going to make our job tougher," said Gen. Keith Alexander, in an interview at the Aspen Security Conference in Colorado.

Alexander said it would be impossible to listen to every phone call and read every e-mail even if the government wanted to, which it does not. Consider, he said, the sheer volume — 114 billion e-mails, 24 billion text messages, and over 12 billion phone calls worldwide every day.

"We're a foreign intelligence agency, we don't have the technical capabilities to do that. You'd have to have AT&T's and everybody else's networks, and we don't. We couldn't compel them to listen to those phone calls. That would require a warrant and a finding of probable cause."

Alexander said the telephone and internet surveillance programs revealed in Snowden's leaks were court-approved but kept secret for a valid reason.

"The purpose of these programs, and the reason we use secrecy, is not to hide from the American people, not to hide it from you, but to hide it from those who walk among you who are trying to kill you."

Since the programs became public, the Obama administration faces growing pressure to reveal more about how the massive programs work. One gathers data on phone numbers dialed and length of calls made in the US, though not call content. The other allows the NSA to monitor overseas e-mails and Internet sites used by suspected terrorists.

Some of the world's top computer and Internet companies are urged the administration to let them disclose the number and scope of the surveillance requests they get from the NSA, information that is classified.

The federal government also faces new lawsuits brought by privacy advocates who seek to restrict the programs or to make more details about them public.

At the Aspen conference, the head of the ACLU said Edward Snowden did America a service.

"Up until Edward Snowden's revelations, the public debate was anemic. There was very little understanding about surveillance and the implications for ordinary Americans. Now it's much more robust," said Anthony Romero, the ACLU's executive director.