Anti-spying protesters outside the US Department of Justice in Washington, DC, earlier this year. Photo: Reuters

The US National Security Agency has built a surveillance system capable of recording ''100 per cent'' of a foreign country's telephone calls, enabling the agency to rewind and review conversations as long as a month after they take place, according to people with direct knowledge of the effort and documents supplied by former contractor Edward Snowden.

A senior manager for the program compares it to a time machine one that can replay the voices from any call without requiring that a person be identified in advance for surveillance.

The National Security Agency building in Fort Meade, Maryland. Photo: AP

The voice interception program, called MYSTIC, began in 2009. Its RETRO tool, short for ''retrospective retrieval'', and related projects reached full capacity against the first target nation in 2011. Planning documents two years later anticipated similar operations elsewhere.

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In the initial deployment, collection systems are recording ''every single'' conversation nationwide, storing billions of them in a 30-day rolling buffer that clears the oldest calls as new ones arrive, according to a classified summary.

The call buffer opens a door ''into the past'', the summary says, enabling users to ''retrieve audio of interest that was not tasked at the time of the original call''. Analysts listen to only a fraction of 1 per cent of the calls, but the absolute numbers are high. Each month, they send millions of voice clippings, or ''cuts'', for processing and long-term storage.

Former NSA contractor Edward Snowden. Photo: Getty Images

At the request of US officials, this article withholdsdetails that could be used to identify the country where the system is being employed or other countries where its use was envisioned.

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NSA surveillance program can retrieve, replay phone calls