Guardian columnist Glenn Greenwald on Sunday revealed that he would be publishing new details that backed up former National Security Agency (NSA) contractor Edwards Snowden's claim that low-level analysts could listen to the phone calls of any American or even read President Barack Obama's emails.

"The story that we've been working on for the last month that we're publishing this week very clearly sets forth what these programs are that NSA analysts -- low-level ones, not just one that work for the NSA, but private contractors like Mr. Snowden -- are able to do," Greenwald told ABC News host George Stephanopoulos. "The NSA has trillions of telephone calls and emails in their databases that they’ve collected over the last several years."

He continued: "And what these programs are, are very simple screens, like the ones that supermarket clerks or shipping and receiving clerks use, where all an analyst has to do is enter an email address or an IP address, and it does two things. It searches that database and lets them listen to the calls or read the emails of everything that the NSA has stored, or look at the browsing histories or Google search terms that you’ve entered, and it also alerts them to any further activity that people connected to that email address or that IP address do in the future.

Greenwald insisted that it was "all done with no need to go to a court, with no need to even get supervisor approval on the part of the analyst."

The columnist admitted that there were "legal constraints for how you can spy on Americans, you can't target them without going to the FISA court."

"But these systems allow analysts to listen to whatever emails they want, whatever telephone calls, browsing histories, Microsoft Word documents," he pointed out. "It’s an incredibly powerful and invasive tool, exactly of the type that Mr. Snowden described."

"NSA officials are going to be testifying before the Senate on Wednesday, and I defy them to deny that these programs work exactly as I just said."