As the National Security Agency (NSA) spying furore rumbles on, the agency has claimed to be looking at only 0.00004 percent of the world's total internet traffic.

In a document (PDF) on the nsa.gov website, the agency said that the internet carries 1,826 Petabytes of information per day, and that its activity "touches" 1.6 percent of that data — approximately 29 petabytes, or 29 million gigabytes, of data each day. Of that number, the agency says 0.025 percent is selected for review.

"The net effect is that NSA analysts look at 0.00004 percent of the world's traffic in conducing their mission — that's less than one part in a million," says the document. "Put another way, if a standard basketball court represented the global communications environment, NSA's collection would be represented by an area smaller than a dime on that basketball court."

Throughout the document, the agency defended its legal basis, and dismissed claims that it was conducting dragnet-style operations, except for instances that fall under the umbrella of counter-terrorism and involve telephone metadata.

Under its Business Records FISA (BR FISA) program, the agency is able to gain access to telephone metadata from US telcos that includes "called and calling telephone numbers and the date, time, and duration of the call — but no user identification, content, or cell site locational data".

"The purpose of this particular collection is to identify the US nexus of a foreign terrorist threat to the homeland," the document said. "The government cannot conduct substantive queries of the bulk records for any purpose other than counter-terrorism."

The agency pointed to the effectiveness of the program in preventing terrorist operations.

"Of the 54 terrorism events recently discussed in public, 13 of them had a homeland nexus, and in 12 of those cases, BR FISA played a role.

"Every search into the BR FISA database is auditable, and all three branches of our government exercise oversight over NSA's use of this authority."

The NSA pointed out that the BR FISA program began in 2006, and has been reauthorised by two different presidential administrations, four congresses, and 14 federal judges.

Despite such programs, the NSA claimed that it is able to conduct itself in a manner that respects civil liberties while maintaining national security.

"We do not need to sacrifice civil liberties for the sake of national security; both are integral to who we are as Americans."

The US agency said it partners with 30 different nations, and is prohibited from using its partner agencies to do work that the NSA itself would find illegal.

"These partnerships are an important part of the US and allied defence against terrorists, cyberthreat actors, and others who threaten our individual and collective security.

"Both parties to these relationships benefit."

The document was created to "address inaccuracies that have appeared in the media" and provide a level of transparency for the agency without endangering sources, methods, or national security.