The White House has defended phone-tapping as a vital tool to combat terrorism but a further wave of revelations yesterday about a vast internet surveillance programme looked certain to trigger fresh outrage.
A spy agency sweep of domestic phone records, reported by Britain's Guardian newspaper on Wednesday, was greeted with anger by civil liberties groups, who decried the programme as "beyond Orwellian".
The controversy looked set to widen as the Washington Post and Guardian reported that the National Security Agency (NSA) had tapped directly into the servers of internet giants - including Microsoft, Google, Facebook and Apple - to obtain videos, photographs and emails.
The phone records programme, which began under President George W. Bush, was detailed in a Guardian report based on a copy of a secret court order requiring telephone provider Verizon to turn over call records for a three-month period ending on July 19.
The fresh revelations yesterday of an NSA programme known as Prism - which gave the intelligence community direct access to the servers of web titans including Microsoft, Yahoo, Google, Facebook, Apple, PalTalk, AOL, Skype and YouTube - will likely generate more controversy.
The Washington Post reported that the National Security Agency and FBI had direct access to servers which allowed them to track an individual's web presence via audio, video, photographs, emails and connection logs.
Some of the biggest firms in Silicon Valley were involved in the programme, reports said.
The leak came from a career intelligence officer "with firsthand experience of these systems and horror at their capabilities", who wanted to expose what he believed was a "gross intrusion on privacy", the Post reported.
Prism was set up in 2007 and has grown "exponentially" to the point where it is now the most prolific contributor to Obama's top-secret daily intelligence briefing.
Advocates say the data, collected by the NSA on phone calls inside and outside the United States, can be crunched to show patterns of communication to alert spy agencies to possible planning for terror attacks.
Senior US officials, while not confirming reports in the Guardian, defended the concept of collecting millions of phone records, and argued the programme was lawful and subject to multiple checks and balances across the Government.
"The top priority of the President of the United States is the national security of the United States. We need to make sure we have the tools we need to confront the threat posed by terrorists," White House spokesman Josh Earnest said.
"What we need to do is balance that priority with the need to protect civil liberties," he said, adding that President Obama welcomed debate on the issue.
Mike Rogers, Republican chairman of the House Permanent Select Committee on Intelligence, said the programme was vitally important.
"Within the last few years this programme was used to stop a terrorist attack in the United States. We know that," Rogers said.
Officials said the programme did not "listen in" on calls or pull the names of those on the line, but simply collated phone numbers, the length of individual calls and other data.
A US official said the programme allows counterterrorism investigators to find out whether suspected terrorists have been in contact with other suspects, particularly people located in the US.
Randy Milch, Verizon's Executive Vice President and General Counsel, said in a message to staff that he was legally forbidden to comment but that any such court order would compel the company to comply.
The revelations ignited new controversy for the White House as it battles claims of harsh treatment toward leakers, accessing phone records of the Associated Press and targeting a Fox News reporter in an intelligence probe.
An NSA phone surveillance programme was first reported during the Bush Administration and formed part of the sweeping anti-terror laws and surveillance structure adopted after the September 11, 2001, attacks.
But the latest revelations are the first sign that the technique is continuing under Obama - though laws authorising such practices had already been reauthorised under the present administration.
"It's a programme in which some untold number of innocent people have been put under the constant surveillance of Government agents," said Jameel Jaffer, deputy legal director of the American Civil Liberties Union.
"It is beyond Orwellian."
The Post revelations on Prism were also reported by the Guardian, which said the programme allowed the intelligence community to collect information such as search history, emails, file transfers and live chats.
Although the leaked document indicated the programme had been sanctioned by the companies involved, several denied any knowledge of it.
Google dismissed suggestions it had opened a "back door" for intelligence agencies.
"Google cares deeply about the security of our users' data.
"We disclose user data to government in accordance with the law, and we review all such requests carefully," the company said in a statement quoted by the Guardian.
The Guardian reported that Microsoft had been part of the programme since 2007, with Apple the most recent addition in October 2012.
Apple later issued a strongly worded statement rejecting all knowledge of the plan.
"We have never heard of Prism. We do not provide any government agency with direct access to our servers, and any government agency requesting customer data must get a court order," Apple spokesman Steve Dowling said.
The claim: The NSA tapped directly into the servers of nine internet firms, including Microsoft, Google, Facebook and Apple, to track people.
The programme: Prism, the single largest source of NSA data.
The companies: Microsoft, Yahoo, Google, Facebook, PalTalk, YouTube, Skype, AOL and Apple.
On the line
The revelation: The United States' National Security Agency is collecting the telephone records of millions of US customers.
The demand: That Verizon, one of the country's largest telecoms providers, give the NSA information on all telephone calls in its systems, both within the US and between the US and other countries on an "ongoing, daily basis" for a three-month period ending on July 19.
The authorisation: A court order granted by the secret Foreign Intelligence Surveillance Court on April 25.