The United States' top intelligence official angrily defended his government's secret monitoring of Internet users, insisting the vast operation is both legal and vital to national security.
James Clapper, the US spy chief pictured here during a Senate committee hearing in Washington on January 31, 2012, lashed out Saturday at what he called "reckless" press disclosures about US intelligence agencies' data mining programs, defending them as tools to fight terror.
Director of National Intelligence James Clapper confirmed that US spy agencies use a system called "PRISM" to gather data trails left by targeted foreign citizens using the Internet outside the United States.
But he said reports by The Guardian and The Washington Post, based on leaked documents, failed to put the program in context, and insisted PRISM is overseen by a secret court under laws approved by the US Congress.
"Over the last week we have seen reckless disclosures of intelligence community measures used to keep Americans safe," Clapper said, dubbing PRISM "one of our most important tools for the protection of the nation's security."
"PRISM is not an undisclosed collection or data mining program," he said.
"It is an internal government computer system to facilitate the government's statutorily authorized collection of foreign intelligence information from electronic communication service providers under court supervision."
President Barack Obama's national security spokesman Ben Rhodes said the administration was investigating whether the leak had put Americans or US interests in danger, implying that legal action may be considered.
"What we're focused on doing right now... is frankly doing an assessment of the damage that's been done to the national security of the United States by the revelations of this information," he said.
The service providers -- Internet titans like Google, Yahoo! and Facebook -- also hit back, insisting they had not given direct access to customer data.
"Press reports that suggest that Google is providing open-ended access to our users' data are false, period," Google's CEO Larry Page and chief legal officer David Drummond said in a message on their official company blog.
"We had not heard of a program called PRISM until yesterday," they said, adding: "We provide user data to governments only in accordance with the law."
Facebook founder Mark Zuckerberg described the press reports as "outrageous," insisting that his firm only provided user information to the authorities when compelled to by law. Yahoo! issued a similar denial.
"The notion that Yahoo! gives any federal agency vast or unfettered access to our users' records is categorically false," general counsel Ron Bell said.
"We do not voluntarily disclose user information. The only disclosures that occur are in response to specific demands."
Under PRISM, which has been running for six years, the US National Security Agency can issue directives to Internet firms demanding access to emails, online chats, pictures, files, videos and more uploaded by foreign users.
The initial press reports that revealed the secret program suggested the NSA had some form of back door access to the servers of firms including Microsoft, Yahoo, Google, Facebook, AOL, Skype, Apple, PalTalk and YouTube.
But Clapper's statement described a system whereby the government must apply to a secret US court for permission to target individuals or entities then issue a request to the service provider.
"The government cannot target anyone under the court approved procedures... unless there is an appropriate, and documented, foreign intelligence purpose for the acquisition," Clapper said.
Such a purpose, he continued, could be "the prevention of terrorism, hostile cyber activities or nuclear proliferation."
He admitted that data on US citizens might be "incidentally intercepted" in the course of targeting a foreign national, but said this would not normally be shared within the intelligence community unless it confirmed a threat.
PRISM was revealed shortly after The Guardian uncovered another intelligence program under which the NSA hoovered up the telephone records of millions of US citizens from the private telecoms provider Verizon.
Obama has defended the phone and Internet data trawls as a "modest encroachment" on privacy needed to keep Americans safe from terrorism.
But civil liberties and privacy groups have raised alarm at both programs, which some have branded "Orwellian" and possibly unconstitutional.
There have also been concerns abroad. British opposition lawmakers have demanded an inquiry into a report the British signals intelligence agency GCHQ received PRISM data from its US ally and used it in reports.