The National Security Agency is harvesting huge numbers of images of people from communications that it intercepts through its global surveillance operations for use in sophisticated facial recognition programs, according to top-secret documents. The spy agency's reliance on facial recognition technology has grown significantly over the last four years as the agency has turned to new software to exploit the flood of images included in emails, text messages, social media, videoconferences and other communications. Agency officials believe that technological advances could revolutionize the way that the N.S.A. finds intelligence targets around the world. The agency's ambitions for this highly sensitive ability and the scale of its effort have not previously been disclosed. The agency intercepts millions of images per day" - including about 55,000 "facial recognition quality images" - which translate into "tremendous untapped potential," according to 2011 documents obtained from the former agency contractor Edward J. Snowden. It is not clear how many people around the world, and how many Americans, might have been caught up in the effort. Neither federal privacy laws nor the nation's surveillance laws provide specific protections for facial images. Civil-liberties advocates and other critics are concerned that the power of the improving technology, used by government and industry, could erode privacy. "Facial recognition can be very invasive," said Alessandro Acquisti, a researcher on facial recognition technology at Carnegie Mellon University.