The National Security Agency is collecting billions of records on the location of mobile phones around the world, The Washington Post reported, citing documents from US intelligence leaker Edward Snowden.
The information is added to a gigantic database that shows the locations of "at least hundreds of millions of cell phones" worldwide, a stunning revelation that suggests the eavesdropping agency has created a mass surveillance tool, according to the Post report.
The report comes six months since the first bombshell leaks from Snowden, a former information technology subcontractor for the NSA who says he spilled secrets to spark public debate on the agency's widespread surveillance activities.
Snowden faces espionage charges but has fled to Russia, where he has been granted asylum.
Of the NSA surveillance programs revealed to date, including spying on foreign leaders and the collection of Internet "meta-data," the geo-location project appears to represent the agency's largest in scale and scope.
The NSA declined to comment on the report when contacted by AFP.
The data is scooped up by tapping into cables that link mobile phone networks -- both American and foreign -- across the globe, the Post said. The location data is pulled in with the help of two unnamed corporate firms, according to leaked documents.
Information from the cell phones of Americans traveling abroad also forms part of the database.
As mobile phones broadcast their locations even when there is no call made or text sent, NSA analysts are able to use mathematical techniques to comb through location data and track patterns of movement over time for a given suspect, it said.
The analytic methods used by the agency to sift through location data are known as CO-TRAVELER, according to the report.
Although the vast majority of mobile phone users are of no interest to the spy agency, the NSA gathers the bulk data to try to track known "intelligence targets" and their unknown associates, the paper said.
The NSA insists it does not intentionally track the location data of Americans, but it ends up receiving details that show the whereabouts of domestic mobile devices "incidentally," wrote the Post, which also quoted intelligence officials.
US officials told the Post that the programs that collect geo-location data are legal and designed only to gather intelligence about foreign militants or other "targets" deemed a threat to the United States.
The volume of information flowing in from the program is "outpacing our ability to ingest, process and store" data, according to a May 2012 internal NSA briefing leaked to the Post.
"The NSA's capabilities to track location are staggering, based on the Snowden documents, and indicate that the agency is able to render most efforts at communications security effectively futile," said the Post article written by Barton Gellman and Ashkan Soltani.
The scale of the program will reinforce long-running concerns raised by civil liberties groups that the NSA's electronic spying poses a serious threat to privacy rights in the United States and abroad.
"It is staggering that a location-tracking program on this scale could be implemented without any public debate, particularly given the substantial number of Americans having their movements recorded by the government," said Catherine Crump, an attorney with the American Civil Liberties Union.
Greg Nojeim, a director at the Center for Democracy and Technology, called for Congress "to finally act to rein in NSA surveillance."
"It's clear the very personal location records of innocent people, including American citizens, are being collected and analyzed in previously unimaginable ways and on a massive scale," it said.