US spies are secretly tapping into the servers of nine Internet giants, including Apple, Facebook, Microsoft and Google, in a vast anti-terror sweep targeting foreigners, reports said on 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 US citizens 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 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 companies 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.

Adamant denials

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 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 US citizens 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.

Reprehensible act

"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.

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, authorized by a top secret court order, as "beyond Orwellian".

The group urged supporters to get angry and "be part of a strong public outcry against this program" by signing a petition.

But Mike Rogers, Republican chairman of the House Permanent Select Committee on Intelligence, said the program 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 program was lawful and subject to multiple checks and balances across the government.

Agence France-Presse