America's National Security Agency has technology that is capable of recording the phone calls of an entire country and replaying them later, a report based on leaked documents said Tuesday.

The Washington Post, citing papers released by the former NSA contractor Edward Snowden, said the eavesdropping agency's equipment functions like a time machine by being able to reach into the past.

The report said the NSA can collect 100 percent of the calls of a target country, reaching as far back as one month with tools called MYSTIC and RETRO.

The leaked documents say the technology can "retrieve audio of interest that was not tasked at the time of the original call."

A classified summary of the program cited by the Post said an NSA collection system can store billions of calls in a 30-day rolling buffer that clears the oldest calls as new ones arrive.

The newspaper said that at the request of US officials it had withheld details that could be used to identify the country where the system is being used, and other nations where it may be used in the future.

If true, the program would be more powerful than any other that the NSA has, allowing the spy agency to tap into the entire network from a country.

Jameel Jaffer of the American Civil Liberties Union called the report "a truly chilling revelation."

"The NSA has always wanted to record everything, and now it has the capacity to do so," Jaffer said in a statement.

"The question now is simply whether we have the political will to impose reasonable limits on the NSA's authority - that is, whether we have the political will to protect our democratic freedoms."

Zeke Johnson at Amnesty International USA said: "The NSA is making George Orwell seem unimaginative. If true, this latest revelation should be a clarion call for reform."

The National Security Agency did not confirm or deny the report but said in a statement that its electronic spying was not "arbitrary," and noted that its methods respect laws and the privacy rights of Americans and foreigners.

The agency also renewed its criticism of the stream of leaks from Snowden.

"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," it said.

Dozens of documents leaked by Snowden have sparked outrage in the United States and abroad about the vast capabilities of the intelligence programs.

US officials have defended the tactics as necessary to thwart terror attacks, though President Barack Obama has ordered reforms for the surveillance programs.

Snowden separately Tuesday promised more sensational revelations about US spying. In a video link to a technology conference in Vancouver, he said "some of the most important reporting to be done is yet to come."

Documents leaked by Snowden in 2013 revealed widespread surveillance of individuals and institutions in the United States and around the world.

Snowden received temporary asylum in Russia in August -- a move that infuriated the United States and was a key factor behind Obama's decision to cancel a summit with Russian President Vladimir Putin last year.