Washington DC, Aug 16, 2013 - President Barack Obama's promises to protect Americans from domestic spying came under fresh scrutiny on Friday, after an internal audit showed the National Security Agency had repeatedly violated privacy rules in its electronic surveillance programs.

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 that 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.

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 lawmakers.

“We don't hide these incidents,” he said.

Defending the NSA's track record, DeLong estimated it had about a .0005 percent 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, lawmakers vowed more hearings to learn the full extent of the privacy violations and rights groups expressed outrage.

“We have previously said that the violations of these laws and rules were more serious than had been acknowledged, and we believe Americans should know that this confirmation is just the tip of a larger iceberg,”

said US senators Ron Wyden and Mark Udall in a joint statement.

The two Democrats said “the American people have a right to know more details about of these violations” and urged the Obama administration to release more information about the programs to allow a public debate.

In a NSA audit dated May 2012, there were 2,776 “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.

“The number of 'compliance incidents' is jaw-dropping,” said Jameel Jaffer, deputy legal director at the American Civil Liberties Union.

“The rules around government surveillance are so permissive that it is difficult to comprehend how the intelligence community could possibly have managed to violate them so often,” Jaffer said.

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. Just as the White House believed it had begun to defuse concerns over domestic spying, the latest leak thrust the issue back in the spotlight.

In Friday's White House statement, deputy press spokesman Josh Earnest added: “The documents demonstrate that the NSA is monitoring, detecting, addressing and reporting compliance incidents. We have been keeping the Congress appropriately informed of compliance issues as they arise.”

The Post also reported that the Foreign Intelligence Surveillance Court (FISC), which is supposed to oversee surveillance activities, lacks the tools to check on the spy agencies and must rely on the services themselves for any information on possible excesses.

“The FISC is forced to rely upon the accuracy of the information that is provided to the Court,” the chief judge of the court, Reggie Walton, said in a statement to the Post.

In some cases, the NSA was less than transparent and withheld details when reporting to the Justice Department and the Office of the National Intelligence Director, according to the Post report.

But DeLong at the NSA said the agency was vigilant about any sign of a breach of privacy regulations.