Top-secret documents on U.S. government surveillance provided by whistleblower and former National Security Agency (NSA) contractor Edward Snowden revealed this week that the agency has been harvesting millions of e-mail addresses from contact lists of users of sites such as Facebook, Gmail, Hotmail and Yahoo.

Documents that Mr. Snowden shared with The Washington Post and remarks from unnamed intelligence officials were said to show that 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.

Reports referenced an internal NSA PowerPoint presentation which suggested that the figures represented a “typical daily intake… [and] correspond to a rate of more than 250 million a year”.

The latest exposé comes in the wake of assurances given by U.S. President Barack Obama in June that the NSA’s e-mail collecting program “does not apply to U.S. citizens”. In its report this week, The Post however quoted two senior U.S. intelligence officials who acknowledged that though the collection of e-mail address lists took place overseas, “it sweeps in the contacts of many Americans”, possible numbering in the “millions or tens of millions”.

Shawn Turner, Spokesman for the Directorate of National Intelligence, which oversees the NSA, reportedly said the Agency was “focused on discovering and developing intelligence about valid foreign intelligence targets like terrorists, human traffickers and drug smugglers. We are not interested in personal information about ordinary Americans”.

However, the PowerPoint presentation shared by Mr. Snowden suggested that the contact lists stored online supplied the NSA with “far richer sources of data than call records alone”, and this data included telephone numbers, street addresses, and business and family information, in addition to basic names and e-mail addresses.

The Agency’s surveillance programmes are in theory operated under the rules prescribed by the Foreign Intelligence Surveillance Act (FISA) Court. However neither the FISA Court not the U.S. Congress was said to have authorised this particular data collection activity, and according to officials the programme may even be “illegal” under U.S. law.