The Washington Post reports the National Security Agency has been recording foreign country's phone calls and listening to the conversations up to one month later.
The paper did not identify the country at the request of government officials.
"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," he said.
But a NSA spokesperson said the agency's collection programs are legal and done for the sake of national security.
"NSA does not conduct signals intelligence collection in any country, or anywhere in the world, unless it is necessary to advance U.S. national security and foreign policy interests and to protect its citizens and the citizens of its allies and partners from harm," NSA spokeswoman Vanee Vines said in an email.
The program is the latest to come to light after leaks by former NSA contractor Edward Snowden.