In a statement to the US media, the NSA Friday acknowledged "very rare instances of willful violations of NSA's authorities have been found" over the past decade.
"NSA takes very seriously allegations of misconduct, and cooperates fully with any investigations responding as appropriate. NSA has zero tolerance for willful violations of the agency's authorities," the agency added.
The agency said the deliberate actions did not violate the 1978 Foreign Intelligence Surveillance Act or the USA Patriot Act, but instead overstepped a 1981 executive order governing US intelligence operations issued by former US president Ronald Reagan.
White House spokesman Josh Earnest reiterated President Barack Obama's assertion Wednesday that there was no domestic surveillance programme going on in the country.
Earnest stressed that the NSA's surveillance efforts was focused on foreign intelligence and domestic information was only occasionally accessed by the agency as "compliance issues" rather than systematic spying on US citizens.
According to special court opinions declassified earlier this week, the NSA had improperly collected 56,000 emails and other communications between Americans annually for three years before the court ruled it unconstitutional.