A secret surveillance system known as XKeyscore allows US intelligence to monitor "nearly everything a typical user does on the Internet," according to leaked documents published Wednesday.
Citing classified documents provided by fugitive intelligence contractor Edward Snowden, British daily the Guardian said the program was the most wide-reaching operated by the National Security Agency (NSA).
The paper said the existence of XKeyscore proves the truth of Snowden's earlier claim, denied by some US officials, that before he left the NSA he could "wiretap anyone, from you or your accountant, to a federal judge or even the president, if I had a personal email."
The White House insisted, however, that access to such tools is only available to those who are assigned to use them and that multiple "checks and balances" are in place to prevent abuse.
"As we've explained and the intelligence community has explained, allegations of widespread, unchecked analyst access to NSA... data are false," White House spokesman Jay Carney said.
On its website, the Guardian published a batch of slides from what appears to be an internal US intelligence training briefing laying out the capabilities of the XKeyscore program.
The paper had blacked out four of the 32 slides because "they reveal details of specific NSA operations," but the remainder of the document lays out the operation of the program in detail.
The slides are marked "Top Secret" and restricted to authorized personnel from the United States, Britain, Canada, Australia and New Zealand. They were produced in 2007 and not due to be declassified until 2032.
According to the slides, XKeyscore allows US spies to monitor in real time the emails, web browsing, Internet searches, social media use and virtually all online activity of a target.
The system's computer infrastructure is based on a "massive distributed Linux cluster" and has 500 servers distributed around the world.
A map included in the briefing suggests that these servers are located on every continent, on the territory of US allies and of rivals like Russia, China and Venezuela.
Where XKeyscore appears to differ from other US surveillance programs that have already been revealed is that it can index and make searchable virtually any online activity.
"No other system performs this on raw unselected bulk traffic," the document boasts.
XKeyscore does not require an intelligence analyst to have a "strong selector" such as an email address to find a target -- agents can work back from a general search to find an individual.
The examples it gives are if someone is using an unusual language for his area, such as German in Pakistan, or using Google Maps to scout targets for attack.
The document boasts that XKeyscore has allowed US agents to capture "over 300 terrorists" but pages that apparently relate to specific operations have been redacted by the Guardian.
The slides also say that XKeyscore is being updated to make it more powerful and faster and to broaden the range of data it can search to include, for example, the EXIF data embedded in digital photographs.
The Guardian interviewed Snowden in June, when the young contractor was in Hong Kong after fleeing his job at the NSA in Hawaii carrying a trove of top secret documents.
The paper did not say why it had not published details of XKeyscore in June, when it revealed the existence of a less capable US surveillance system known as Prism.
But the leak coincided with a hearing of the US Senate judiciary committee at which top intelligence officials are to be grilled by lawmakers concerned that spy agencies have exceeded the law.
It is illegal in the United States for intelligence agencies to monitor US citizens without a court order, but Snowden's leaks have shown that Americans are regularly caught up in monitoring sweeps.
The revelations have also embarrassed Washington abroad, where some have been shocked by the extent of US Internet surveillance.