The head of the National Security Agency explained the agency's use of facial recognition technology on Tuesday, pushing back on a report that claimed the NSA is collecting millions of images of Americans as part of a surveillance database.

During a cyber security conference in Washington, Adm. Michael Rogers acknowledged his agency is using facial recognition technology, but he emphasized the strict legal limits on the application of that the technology to U.S. citizens, according to Bloomberg News.

The defense was prompted by a report last Saturday in the New York Times that the spy agency is intercepting "millions of images per day" of people around the world. The report was based on documents obtained by Edward Snowden, a former NSA contractor who fled the country last June after exposing several highly classified government surveillance programs.

Get Breaking News Delivered to Your Inbox

The documents provided by Snowden, according to the Times, show that the agency has broadened its focus beyond written and oral communication to include facial images, fingerprints, and other personal identifiers that could help them keep tabs on suspected terrorists or other surveillance targets.

On Tuesday, Rogers said the legal limits that govern other forms of information intercepted by the agency apply to the facial images as well.

"We do not do this in some unilateral basis against U.S. citizens," Rogers said, according to Bloomberg News. "We have very specific restrictions when it comes to U.S. persons."

"In broad terms, we have to stop what we're doing if we come to the realization that somebody we're monitoring or tracking has a U.S. connection that we were unaware of," he added. "We have to assess the situation and if we think there is a legal basis for this and we have to get the legal authority or justification."

An NSA spokeswoman told the Times that the NSA is required to obtain judicial approval to browse pictures of U.S. citizens obtained by the agency, much as it would be required to get court approval to sift through their telephone records or emails.

Still, this newest revelation -- the latest in a series of rolling disclosures from Snowden about the vast capabilities of the U.S. intelligence community -- has civil liberties advocates worried about a destruction of privacy.

The controversy sown by Snowden's previous disclosures has prompted calls for reform from activists and lawmakers. In May, the House approved a bill that would prohibit the spy agency's bulk collection of metadata from domestic phone calls. Under that proposal, the phone companies would retain the records themselves, and the NSA would be forced to obtain a court order to direct the companies to search the records. The Senate has not yet voted on the legislation.