WASHINGTON — The National Security Agency has been recording all of a foreign country’s phone calls, then listening to the conversations up to a month later, The Washington Post reported Tuesday.

At the request of U.S. officials, the Post said it would not identify the targeted country or other countries where the program’s use was envisioned by officials.

The program is the latest revelation from a trove of classified documents that former NSA systems analyst Edward Snowden leaked to certain news organizations last year. Most of those documents have described the U.S. collecting massive amounts of data and text. This program is different in that it records phone calls.

This NSA program dates to 2009 and is called MYSTIC, according to documents obtained by the Post. It is used to intercept conversations in one specific country, but documents show the NSA intends to use it in other countries, the Post said.

It records all conversations across the unidentified foreign country and stores billions of them for 30 days. The program wasn’t fully operational until 2011. One of the program’s senior managers told the Post that MYSTIC is comparable to a time machine, meaning voices from any call can be replayed without requiring the NSA to identify a person before the conversations are collected.

The conversations swept up likely include those of Americans who make calls to or from the targeted country. Civil libertarians are concerned that this program and others like it will target other countries and that the NSA will eventually hold the data longer than what was defined its original charter and use it for other reasons.

“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,” Jameel Jaffer, American Civil Liberties Union deputy legal director, said in a statement. “The NSA has always wanted to record everything, and now it has the capacity to do so.”