The National Security Agency is gathering email and instant messenger contact lists from hundreds of millions of ordinary citizens worldwide, many of them Americans, The Washington Post reported late 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 figures, described as a typical daily intake in the document, correspond to a rate of more than 250 million a year, according to the report, which was published on the newspaper's website.
The NSA declined to confirm the specific allegations in the Post report but defended its surveillance activities as legal and respectful of privacy rights.
The agency has come under fire following revelations about vast efforts to collect data on Americans, but it has mostly acknowledged the accuracy of leaks from Snowden while seeking to play down their significance.
The Snowden affair has not only complicated diplomacy but embarrassed the Internet and telecom sector, with some companies accused of betraying their customers by cooperating with government spying.
Russia has granted Edward Snowden one year's asylum but the United States wants him to be extradited to face espionage charges over his leaking of sensational details of US surveillance programmes at home and abroad.