President Barack Obama's promises to protect Americans from domestic spying has come under fresh scrutiny after an internal audit showed the National Security Agency (NSA) had repeatedly violated privacy rules in its electronic surveillance.
The revelations appeared to challenge Obama's reassurances that strict oversight of NSA snooping had prevented abuses.
The Washington Post, citing NSA documents and the audit, reported the eavesdropping service had breached privacy restrictions thousands of times and in some cases withheld details from other government departments.
The Post report quoted documents leaked from Edward Snowden, the former NSA contractor who has exposed the massive scale of America's surveillance of phone records and internet traffic in a series of dramatic disclosures to the media.
Snowden, who describes himself as a whistleblower, has been granted asylum in Russia, despite appeals from Washington for his extradition on espionage charges.
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The NSA did not deny the privacy violations but offered a more positive assessment, saying the mistakes were unintentional and "miniscule" in number given the vast amount of data streaming through the spy agency.
"These are not willful violations. These are mistakes," John DeLong, NSA's director of compliance, told reporters in a teleconference, noting that the agency promptly reports surveillance errors to a court, other government departments and to politicians.
"We don't hide these incidents."
Defending the NSA's track record, DeLong estimated it had about a .0005 per cent error rate, with roughly 100 mistakes out of 20 million queries a month.
A statement issued later from the White House said that Obama "has long advocated greater transparency" and stronger oversight of the intelligence programs to "strike the right balance" between protecting national security and the privacy of citizens.
Although the NSA tried to play down the report, politicians vowed more hearings to learn the full extent of the privacy violations and rights groups expressed outrage.
In a NSA audit dated May 2012, there were 2776 "incidents" over the previous year in which the agency exceeded its authority in the collection, storage and distribution of communications, according to the Post.
Most of the cases were unintentional but in at least one instance, the agency violated a court order.
Obama and other top officials have insisted that Americans' privacy rights have been safeguarded and that sweeping surveillance powers have not been abused in the search for terror suspects.
