President Barack Obama has staunchly defended US government programmes conducting surveillance of Americans' phone and Internet activity, insisting that they were conducted with broad safeguards to protect against abuse.
"Nobody is listening to your telephone calls. That's not what this programme is about," Obama told reporters on a visit to California's Silicon Valley on Friday.
He insisted that the surveillance programmes struck the right balance between keeping Americans safe from terrorist attack and protecting their privacy.
On Saturday, the Guardian reported from a secret directive that Obama has ordered his senior national security and intelligence officials to draw up a list of potential overseas targets for US cyberattacks.
His comments follow news that the National Security Agency (NSA) and the Federal Bureau of Investigation (FBI) are tapping directly into the central servers of nine leading US internet companies to spy on foreigners.
According to the Washington Post newspaper, even though US citizens were not the intended targets of surveillance, this has allowed investigators to examine emails, photos and other documents of tens of millions of Americans that can be used to track people and their contacts over time.
In a statement on Thursday, James Clapper, director of national intelligence, said the Post article contains "numerous inaccuracies".
Clapper said while the programme, code-named PRISM, collects information concerning "non-US" persons located outside the country, it cannot be used for any citizens or anyone within the US.
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"Information collected under this programme is among the most important and valuable foreign intelligence information we collect, and is used to protect our nation from a wide variety of threats," he said.
"The unauthorised disclosure of information about this important and entirely legal programme is reprehensible and risks important protections for the security of Americans."
The highly classified national security programme had not been disclosed publicly before.
A US government source, who was not authorised to comment publicly on the programme, confirmed its existence to the Reuters news agency late on Thursday.
Clapper said PRISM, referred to as Section 702 of the Foreign Intelligence Surveillance Act, was recently reauthorised by Congress after "extensive hearings and debate".
The programme's participants, the Post said, include Microsoft, Yahoo, Google, Facebook, PalTalk, AOL, Skype, YouTube and Apple.
The report came on the same day that another newspaper, Britain's Guardian, revealed that the US government is collecting telephone records of millions of Americans as part of US national security efforts.
In Washington, the Guardian report fuelled an ongoing debate over whether the US government is violating citizens' privacy rights while trying to protect them from attacks.
That debate is sure to escalate with the Post's report, which said the NSA and FBI are extracting audio, video, photographs, e-mails, documents and connection logs to build a database involving trillions of communications by Americans.
Al Jazeera's Patty Culhane, reporting from Washington, said that there have been many news stories over the years about intelligence gathering but this is the first time a particular story is leading all local newscasts.
She said change is possible and the programme could end if there is a large enough outcry.
In statements released late on Thursday, Google, Apple, Yahoo and Facebook denied that the government had "direct access" to their central servers.
"Google cares deeply about the security of our users' data. We disclose user data to government in accordance with the law, and we review all such requests carefully.
"From time to time, people allege that we have created a government 'back door' into our systems, but Google does not have a 'back door' for the government to access private user data," the company said in a statement.
In a statement, Facebook's Chief Security Officer Joe Sullivan said: "Protecting the privacy of our users and their data is a top priority for Facebook,"
"We do not provide any government organisation with direct access to Facebook servers,"
US Attorney General Eric Holder said on Thursday that members of Congress were fully briefed on the intelligence-gathering programme that included the daily collection of telephone records from Verizon Communications.