Mystic is the only known US surveillance programme to capture every call across a country’s telephone network.
The US National Security Agency (NSA) has reportedly been recording every single phone call in an undisclosed foreign country via a surveillance system that allows it to replay recordings as old 30 day.
The Washington Post revealed that details were keeping the recordings under wrap at the request of US officials, which could identify the country in which the system is being deployed.
Known as MYSTIC, millions of voice "cuts" are extracted for long-term storage since 2009, according to the latest documents from NSA whistleblower Edward Snowden.
The programme has a retrospective retrieval search tool, called RETRO, to query data on the vast system and replay the content of calls.
Condemning the MYSTIC programme, the American Civil Liberties Union official Jameel Jaffer said in a statement: "This is a truly chilling revelation, and it’s one that underscores how high the stakes are in the debate we’re now having about bulk surveillance."
"The NSA has always wanted to record everything, and now it has the capacity to do so."
The US intelligence agency had reportedly pretended to be Facebook to inject malware ‘implants’ on computers and networks and snoop on them via automated systems since 2010.
Last week, the US District Judge Jeffrey White in San Francisco ordered the agency to retain millions of telephone records gathered over five years.