The National Security Agency is gathering email and instant messenger contact lists from millions of citizens worldwide, The Washington Post reported on Monday. The US agency's data collection program harvests the data from address books and "buddy lists", the newspaper said, citing senior intelligence officials and top secret documents provided by the fugitive NSA contractor Edward Snowden.

During a single day last year, the NSA's Special Source Operations branch collected 444,743 e-mail address books from Yahoo, 105,068 from Hotmail, 82,857 from Facebook, 33,697 from Gmail and 22,881 from unspecified other providers, the Post said, according to an internal NSA PowerPoint presentation.

The NSA defended its surveillance activities as legal and respectful of privacy rights.