The National Security Agency says its tracking of mobile phones overseas is legally authorised under a sweeping US presidential order.
The distinction means the extraordinary surveillance program is not overseen by a secretive US intelligence court but is regulated by some US politicians, Obama administration insiders and inspectors general.
Documents obtained from former NSA contractor Edward Snowden showed that the NSA gathers as many as 5 billion records every day about the location data for hundreds of millions of mobile phones worldwide by tapping into cables that carry international mobile phone traffic.
The Washington Post said the collection inadvertently scoops up an unknown amount of US data as well.
The NSA said Friday it was not tracking every foreign phone call and said it takes measures to limit how much US data is collected.
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The NSA has declined to provide any estimates about the number of Americans whose mobile phones it has tracked because they were travelling overseas or their data was irrevocably included in information about foreigners' mobile phones.
"It is not ubiquitous," NSA spokeswoman Vanee Vines said in a statement. "NSA does not know and cannot track the location of every cellphone."
Vines said the collection of the global mobile phone location data is carried out under the White House order that governs all US espionage, known as Executive Order 12333.
That means congressional committees and relevant inspectors general can oversee the program, but the secret court established under the Foreign Intelligence Surveillance Act would not.
A frequent justification for the NSA programs by President Barack Obama and top US intelligence officials is that they are overseen by all three branches of government.
"The NSA claims its collection is incidental, but there is no question it's deliberately engaging in the mass collection of cellphone location data that it knows will inevitably sweep up information on a huge number of innocent Americans," said Catherine Crump, American Civil Liberties Union staff attorney, in a statement.
"And, all of this is happening without any supervision by a court."
