The Obama administration late Thursday acknowledged a top-secret National Security Agency (NSA) program that collects information on Internet users.
The government insisted the program is aimed only at people outside of the United States and is tremendously valuable in stopping terrorism.
Director of National Intelligence James Clapper called the leak of information about the program's existence "reprehensible."
Separately, an administration official said: "Information collected under this program is among the most important and valuable intelligence information we collect and is used to protect our nation from a wide variety of threats."
The Guardian and The Washington Post reported on Thursday that the NSA is running an operation called PRISM that taps directly into the central servers of nine U.S. Internet companies: Microsoft, Yahoo, Google, Facebook, AOL, Skype, YouTube, Apple and PalTalk.
Clapper did not directly reference PRISM in his statement, saying only that the two media reports “refer to collection of communications pursuant to Section 702 of the Foreign Intelligence Surveillance Act” and contain “numerous inaccuracies.”
He noted that Section 702 was recently reauthorized by Congress "after extensive hearings and debate."
Many of the tech companies identified as participating in the program have denied knowledge of the surveillance.
"We have never heard of PRISM," Steve Dowling, an Apple spokesman, said in an emailed statement. "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."
"We do not provide any government organization with direct access to Facebook servers," Joe Sullivan, Facebook's chief security officer, said. "When Facebook is asked for data or information about specific individuals, we carefully scrutinize any such request for compliance with all applicable laws and provide information only to the extent required by law."
The PRISM program gives the government access to user's emails, video chats, audio, photographs and documents, the newspapers reported.
News of PRISM came on the heels of a separate report that the NSA has been collecting the phone records of millions of people in the United States. But unlike the court order for phone records, PRISM reportedly gives the NSA access to the actual contents of users' communications.
The government official said Congress authorized the program under Section 702 of the Foreign Intelligence Surveillance Act, which does not allow the targeting of any U.S. citizen or person in the United States.
"The program is subject to oversight by the Foreign Intelligence Surveillance Court, the Executive Branch and Congress," the official said. "It involves extensive procedures, specifically approved by the court, to ensure that only non-U.S. persons outside the U.S. are targeted, and that minimize the acquisition, retention and dissemination of incidentally acquired information about U.S. persons."
According to The Washington Post, analysts use "selectors" to determine with "51 percent confidence" that a target is foreign before accessing the information.
Training materials obtained by the Post instruct analysts to document accidentally collected U.S. content in quarterly reports, but explain that “it’s nothing to worry about.”
— This report was last updated at 10:24 a.m. on Friday.