According to new reports, the US government has built a system that can record every phone call made over a month in an undisclosed foreign country.
The National Security Agency (NSA) program was created in 2009, the Washington Post reported.
Fugitive Edward Snowden, who leaked details of the system, promised more revelations.
Civil liberties groups called the report “chilling”, but US officials would not comment.
An NSA cover slide used for an internal briefing on the system, known as Mystic, shows a cartoon wizard wielding a staff with a mobile phone at the top.
Mystic is the only known US surveillance program to capture every single call across a nation’s telephone network, according to the Washington Post.
The NSA has built a system that can record every phone call made over a month in an undisclosed foreign country
The newspaper said that, at the request of US authorities, it would not name the foreign country, or others where the system’s use was envisaged.
It reported that a classified summary of the system suggested billions of conversations were being captured in a 30-day rolling buffer that clears out the oldest calls as new ones are made.
When asked about the report at his daily briefing, White House spokesman Jay Carney said: “We don’t, as a general rule, comment on every specific allegation or report.”
But civil liberties activists said it was “a truly chilling revelation”.
“It’s one that underscores how high the stakes are in the debate we’re now having about bulk surveillance,” Jameel Jaffer, of the American Civil Liberties Union, told Reuters news agency.
Meanwhile, Edward Snowden appeared on Tuesday in the form of a remotely controlled robot at the influential TED conference in Vancouver, Canada.
“There are absolutely more revelations to come,” said Edward Snowden, who fled to Russia last year.
“Some of the most important reporting to be done is yet to come.”
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Set up by President Harry S. Truman in 1953, the National Security Agency (NSA) is the eyes and ears of America across the globe, intercepting 1.7 billion emails, phone calls a day.
The NSA is a secretive body that serves the military and intelligence communities by collecting all forms of foreign communications to prevent attacks on the US.
The agency was prohibited by law from intercepting domestic communications without a warrant until George W. Bush issued a caveat in the wake of 9/11 under the controversial “terrorist surveillance program”.
Nonetheless, over the years the NSA has been engulfed in a number of wiretapping scandals.
President Harry S. Truman set up the National Security Agency in 1953
They include President Richard Nixon’s illegal wiretapping, through the NSA, of five members of his national security staff, two newsmen, and a staffer at the Department of Defense in a bid to uncover who was leaking information about his plans for the Vietnam War.
In 2005 it was revealed George W. Bush had authorized the NSA to eavesdrop on Americans calling abroad without warrants in a bid to thwart terrorism. He strenuously denied the allegations until he finally conceded he had committed an impeachable offense.
In 2009, under President Barack Obama, the US Department of Justice acknowledged the NSA had gone beyond its remit in tapping the phonelines of American citizens, including a Congressman but claimed that the acts were unintentional and had since been rectified.
Last month, it was accused of building an $1.2 billion cyber base to keep tabs on American citizens.
The state-of-the-art data centre in the Utah desert – codenamed Bumblehive – is intended to bolster online security efforts.
But former employees say it could be used to monitor people’s private emails.
The NSA branded the allegations “unfounded”, adding that it remained “unwavering” in its respect for U.S. laws and American citizens’ civil liberties, and noted that it was subject to broad oversight by all three branches of government.