The US National Security Agency collects hundreds of millions of contact lists from personal email and instant messaging accounts.
The collection program intercepts email address books and "buddy lists" from instant messaging services as they move across global data links, include many from Americans, the newspaper said in an article posted on its website yesterday, citing senior intelligence officials and documents provided by former NSA contractor Edward Snowden.
The Post said analysing that data lets the NSA search for connections and map relationships among foreign intelligence targets. The data collection takes place outside the USA, but sweeps in the contacts of many Americans, the report said, citing two senior US intelligence officials.
A spokesman for the Office of the Director of National Intelligence, which oversees the NSA, said the agency is focused on discovering and developing intelligence about foreign intelligence targets.
"We are not interested in personal information about ordinary Americans," he told the Post.
Snowden's revelations about the reach and methods of the NSA, including the monitoring of vast volumes of Internet traffic and phone records, have upset US allies from Germany to Brazil. Admirers call him a human rights champion and critics denounce him as a traitor.
The 30-year-old is now living in a secret location in Russia, beyond the reach of US authorities who want him on espionage charges because he leaked the details of top-secret electronic spying programs to the media.
He travelled to Hong Kong in May and later, under pressure from China, flew to Moscow, where he has been granted a year's asylum.