The US National Security Agency is scooping up large quantities of images of people for use in facial recognition programs, the New York Times reported Sunday, citing top secret documents.

The Times said documents, which were obtained from fugitive former US intelligence analyst Edward Snowden, show a significant increase in reliance on facial recognition technology at the agency over the past four years.

The report said the NSA was using new software to exploit a flood of images included in intercepted emails, text messages, social media posts, video conferences and other communications.

It cited leaked 2011 documents as saying the NSA intercepts "millions of images per day," including 55,000 "facial recognition quality images."

The images represented "tremendous untapped potential," according to the report, which said NSA officials believe advances in technology could revolutionize the way the agency finds intelligence targets.

"It's not just the traditional communications we're after: It's taking a full-arsenal approach that digitally exploits the clues a target leaves behind in their regular activities on the net to compile biographic and biometric information" that can help "implement precision targeting," a 2010 document quoted by the newspaper said.

The Times said it wasn't clear how many people, including how many Americans, had been caught up in the effort, but noted that neither US privacy laws nor US surveillance laws provide specific protections for facial images.

A NSA spokeswoman said, however, that the agency would be required to get court approval for imagery of Americans it collects through its surveillance programs.

The agency has been at the center of controversy over the scope of its global electronic surveillance program since they were first revealed by Snowden in June 2013.

The former intelligence contractor is in Russia, where he was granted temporary political asylum last year.