The NSA is collecting the facial images of countless Americans to be used in its facial recognition software, according to confidential government documents obtained by The New York Times.

Using global surveillance technology, the National Security Agency gathers some 55,000 "facial recognition quality images" from daily online activities, including video chats, social media and email, the newspaper reported.

The facial images are part of "millions of images" the agency collects "per day." The 2011 documents, leaked by former NSA contractor and whistleblower Edward Snowden, outline the agency's intensified efforts to keep tabs on alleged terrorists around the world.

"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," according to a 2010 document. Such info can help "implement precision targeting."

It is not known exactly how many people in the U.S. and abroad are included in the image gathering program, the newspaper reported.

Other government agencies, including the FBI, already use facial recognition software. But the NSA outperforms other agencies in terms of access to private communications.

"We would not be doing our job if we didn't seek ways to continuously improve the precision of signals intelligence activities- aiming to counteract the efforts of valid foreign intelligence targets to disguise themselves or conceal plans to harm the United States and its allies," NSA spokesman Vanee M. Vines told The NY Times.

Opponents say the NSA, which already defended itself for monitoring the public's Internet and phone activities, is infringing on society's civil liberties.

"Facial recognition can be very invasive," Alessandro Acquisti, facial recognition technology researcher at Carnegie Mellon University, told the newspaper. "There are still technical limitations on it, but the computational power keeps growing, and the databases keep growing, and the algorithms keep improving."