The U.S. National Security Agency has been collecting millions of contact lists from personal email and instant messaging accounts around the world, including many from Americans.
The "Washington Post" reported the revelation late on October 14. Its report is based on secret documents provided by the former U.S. intelligence contractor Edward Snowden.
The Post said analyzing that data lets the NSA search for connections and map relationships among foreign intelligence targets.
A spokesman for the national intelligence director's office, which oversees the NSA, told the Post that the agency was seeking intelligence on valid targets and was not interested in personal information from ordinary Americans.
Snowden's revelations have embarrassed Washington and angered many of its allies. He is now living in Russia, where he has been granted temporary asylum. Washington wants to try him for revealing classified information.
Based on reporting by AP and Reuters