US spies are capturing "millions of images a day" of people for use in facial recognition programmes, documents leaked by the whistle-blower Edward Snowden show.

The government spy agency, the NSA, was scooping up the images from sources such as social media, private emails and video conferencing, a report in the New York Times said on Sunday.

The report said the NSA was using new software to exploit a flood of images, and cited documents from 2011 that stated 55,000 of the millions of images captured a day were "facial recognition quality images".

The images represented "tremendous untapped potential", the documents 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.

An NSA spokeswoman said, however, that the agency would be required to get court approval for imagery of Americans it collects through its surveillance programmes. The details of any approval would also be a state secret.

At a rate of 55,000 images a day, it would take several years to collect the facial features of every adult American.

The agency has been at the centre of controversy over the scope of its global electronic spying since it was first revealed by Snowden in June 2013.

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

AFP