Washington - US spies are secretly tapping into servers of nine internet giants including Apple, Facebook, Microsoft and Google in a vast anti-terror sweep targeting foreigners, explosive reports said on Thursday.
Stung by a dizzying 24 hours of revelations on covert programmes, the top US spy James Clapper issued a blistering rebuttal of the reports and warned leaks over a separate programme to mine domestic phone records hurt US security.
The White House, facing a fast-escalating controversy over the scale and scope of secret surveillance programmes meanwhile, denied spying on Americans but insisted it must use every tool available to keep the US homeland safe.
The Washington Post, citing a career intelligence officer, said the National Security Agency (NSA) had direct access to internet firm servers, to track an individual's web presence via audio, video, photographs and e-mails.
Some of the biggest firms in Silicon Valley were caught up in the program, known as Prism, including Microsoft, Yahoo, Google, Facebook, Apple, PalTalk, AOL, Skype and YouTube, the reports said.
The paper said the leak came from a career intelligence officer "with firsthand experience of these systems and horror at their capabilities".
"They quite literally can watch your ideas form as you type," the officer was quoted as saying.
Internet giants however denied opening their doors for US spy agencies.
"We have never heard of Prism," said Apple spokesperson Steve Dowling.
"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."
Facebook's chief security officer Joe Sullivan said the huge social network did not provide any access to government organisations.
Google and Microsoft were also adamant that they only disclose what is legally demanded.
In response to the reports, also carried by Britain's Guardian newspaper, the White House said Americans were not being spied on, but did not deny the programme existed.
"It involves extensive procedures, specifically approved by the court, to ensure that only non-US persons outside the US are targeted, and that minimise the acquisition, retention and dissemination of incidentally acquired information about US persons," the official added.
Congress recently re-authorised the programme under Section 702 of the Foreign Intelligence Surveillance Act "after extensive hearings and debate", the official noted.
As a damage control operation gathered pace, Clapper warned data collected under the programme was "among the most important and valuable foreign intelligence information we collect".
"The unauthorised disclosure of information about this important and entirely legal programme is reprehensible and risks important protections for the security of Americans."
Claims of the internet spy operation broke as Washington reeled from a Guardian newspaper report on Wednesday detailing an apparent operation by the NSA to capture millions of domestic phone records.
The American Civil Liberties Union branded the program, authorised by a top secret court order, as "beyond Orwellian".
But Mike Rogers, Republican chair of the House Permanent Select Committee on Intelligence, said the programme had stopped a terrorist attack on US soil.
Advocates say the data, collected on calls inside and outside the US by the NSA, 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 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 spokesperson Josh Earnest said.
But he said Obama welcomed a debate on the balance between civil liberties and security.
Officials said the program 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.
Clapper, the Director of National Intelligence, said the Guardian report discussing the programme, based on a secret court document ordering phone provider Verizon to turn over records, was "misleading".
He said he had ordered the declassification of certain information about the program so Americans could understand the limits of its use.
Randy Milch, Verizon's Executive Vice President and General Counsel, said in a message to staff he was legally forbidden to comment but that any such court order would compel the company to comply.
An NSA phone surveillance programme was first reported during the Bush administration and formed part of sweeping anti-terror laws and a surveillance structure adopted after the 11 September 2001 attacks.