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 Thursday.
Stung by a dizzying 24 hours of revelations on covert programs, the top US spy James Clapper slammed disclosure of information about the scheme, and warned that leaks about a separate program to mine domestic phone records hurt US national security.
The White House, meanwhile, facing a fast-escalating controversy over the scale and scope of secret surveillance programs, 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 emails.
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 spokesman 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 organizations.
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 program existed.
"It involves extensive procedures, specifically approved by the court, to ensure that only non-US persons outside the US are targeted, and that minimize the acquisition, retention and dissemination of incidentally acquired information about US persons," the official said.
Congress recently reauthorized the program under Section 702 of the Foreign Intelligence Surveillance Act "after extensive hearings and debate," the official added.
As a damage control operation gathered pace, Clapper warned that data gathered under the program was "among the most important and valuable foreign intelligence information we collect."
"The unauthorized disclosure of information about this important and entirely legal program is reprehensible and risks important protections for the security of Americans," Clapper said.