The United States government has reportedly built a system that can record every phone call made over a month in an undisclosed foreign country.
The National Security Agency programme was created in 2009. It allows calls to be accessed for 30 days and reached full capacity of monitoring billions of phone calls in 2011, the Washington Post reports.
Fugitive intelligence contractor Edward Snowden, who leaked details of the system and is now in Russia, promised more revelations. He appeared on Tuesday in the form of a remotely controlled robot at the influential TED conference in Vancouver, Canada, the BBC reports.
Civil liberties groups called the report "chilling", but US officials would not comment on Tuesday.
Mystic is the only known US surveillance programme to capture every single call across a nation's telephone network, according to the Post.
The paper said that, at the request of US authorities, it would not name the foreign country, or others where the system's use was envisaged.
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."