WASHINGTON — The U.S. National Security Agency has created a surveillance system that is recording all the phone calls in an undisclosed foreign country, allowing it to play back any conversation up to 30 days later, the Washington Post reported on Tuesday.

The newspaper cited unnamed sources with direct knowledge of the system as well as documents supplied by former NSA contractor Edward Snowden, who since last year has leaked extensive data revealing sweeping American spying activities.

The newspaper said that at the request of U.S. officials, it was withholding details that could be used to identify the nation where the system is being used or others where it might be used in the future. The Post cited documents that envisioned similar U.S. spying operations in other nations.

The voice interception program is known as MYSTIC and started in 2009, with its “retrospective retrieval” capability, called RETRO, reaching full strength in 2011 against the first target nation, the Post reported.

A classified summary of the system said the collection effort was recording “every single” conversation nationwide in the first target country, storing billions of conversations in a 30-day rolling buffer that clears out the oldest calls as new ones are made, the Post reported.

A senior manager for the program likened it to a time machine that can replay voices from any phone call without the need to identify a person for spying in advance, the newspaper reported.

The Post said that no other disclosed NSA program captures a nation’s telephone network in its entirety.

Current and former U.S. officials quoted anonymously by the Post said large numbers of conversations involving Americans would be gathered using the system.

White House spokesman Jay Carney, at his regular news briefing on Tuesday, sidestepped a question about the Post article, saying that “we don’t, as a general rule, comment on every specific allegation or report.”

“We make clear what activity the NSA and … our intelligence community engages in, and the fact that they are bound by our laws and the oversight of three branches of government,” Carney told reporters.

Carney also noted that President Barack Obama announced a series of steps in January to “significantly reform our activity.