The United States' top intelligence official angrily defended his government's secret monitoring of Internet users Saturday, insisting the vast operation is both legal and vital to national security.
Director of National Intelligence James Clapper confirmed that US spy agencies use a system called "PRISM" to gather up the data left by 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."
The service providers -- identified in the reports as Internet titans like Google, Microsoft, Apple and Facebook -- also hit back at the reports, insisting they had not given US spies direct access to their customers' 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.
"Our legal team reviews each and every request, and frequently pushes back when requests are overly broad or don't follow the correct process," they said.
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 and videos uploaded by targeted foreign users.
The initial reports that revealed the secret program suggested that 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 the companies firmly deny this and 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" and the foreign target must be reasonably believed to be outside the United States.
"We cannot target even foreign persons overseas without a valid foreign intelligence purpose," he said.
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.
The administration has also defended that program, and Google insisted its sometimes reluctant cooperation with requests for data is not on the same scale.
"Until this week's reports, we had never heard of the broad type of order that Verizon received -- an order that appears to have required them to hand over millions of users' call records," Page and Drummond wrote.
"We were very surprised to learn that such broad orders exist. Any suggestion that Google is disclosing information about our users' Internet activity on such a scale is completely false."
President Barack 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.