The US National Security Agency is recording every single phone call in one particular country, with the agency able to rewind and review conversations up to a month after they take place, according to people with direct knowledge of the effort and documents supplied by former contractor Edward Snowden.

The programme, 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.

At the request of US officials, The Washington Post is withholding details that could be used to identify the country or other countries where the system’s use is envisioned.

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.

Analysts listen to only a tiny fraction 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.

No other NSA programme disclosed to date has swallowed a nation’s telephone network whole.

In a statement, Caitlin Hayden, spokeswoman for the NSA, declined to comment on “specific alleged intelligence activities”.

Speaking generally, she said “new or emerging threats” were “often hidden within the large and complex system of modern global communications, and the United States must consequently collect signals intelligence in bulk in certain circumstances in order to identify these threats”.

NSA spokeswoman Vanee Vines, in an e-mailed statement, said that “continuous and selective reporting of specific techniques and tools used for legitimate US foreign intelligence activities is highly detrimental to the national security of the United States and of our allies, and places at risk those we are sworn to protect”.

Some of the documents provided by Snowden suggest that high-volume eavesdropping may soon be extended to other countries, if it has not been already. The Retro tool was built three years ago as a “unique one-off capability”, but last year’s secret intelligence budget named five more countries for which the Mystic programme provided “comprehensive metadata access and content”, with a sixth to be in place by last October.

Ubiquitous voice surveillance, even overseas, pulls in a great deal of content from Americans who telephone, visit and work in the target country.

It may also be seen as inconsistent with US President Barack Obama’s January 17 pledge “that the United States is not spying on ordinary people who don’t threaten our national security”, regardless of nationality.

The emblem of the Mystic programme depicts a cartoon wizard with a telephone-headed staff. Among the agency’s bulk collection programmes disclosed over the past year, its focus on the spoken word is unique.

Most of the programmes have involved the bulk collection of either metadata – which does not include content – or text, such as e-mail address books.

In the first year of its deployment, a programme officer wrote that the project “has long since reached the point where it was collecting and sending home far more than the bandwidth could handle”.

Because of similar capacity limits across a range of collection programmes, the NSA is leaping forward with cloud-based collection systems and a gargantuan new “mission data repository” in Utah.

According to its overview briefing, the Utah facility is designed “to cope with the vast increases in digital data that have accompanied the rise of the global network”.