The Obama administration faced a backlash from congressional Democrats on Friday following revelations that the National Security Agency broke privacy rules and overstepped its authority thousands of times since 2008.

The details were reported late Thursday in The Washington Post, based on an audit and other secret documents provided by NSA leaker Edward Snowden. The report challenged claims by President Obama just last week that the NSA was not abusing its authority, and complicated his effort to reassure Americans and Congress that -- with a little more oversight and transparency -- the surveillance programs are nothing to be worried about.

House Democratic Leader Nancy Pelosi called the latest reports "extremely disturbing."

Rep. Mike Thompson, D-Calif., a member of the House Intelligence Committee, said: "Reports that the NSA repeatedly overstepped its legal boundaries, broke privacy regulations and attempted to shield required disclosure of violations are outrageous, inappropriate and must be addressed."

Senior lawmakers said they had been unaware of the audit until they read the news on Friday.

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Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., said he planned to hold another hearing in the wake of the report.

"The American people rely on the intelligence community to provide forthright and complete information so that Congress and the courts can properly conduct oversight. I remain concerned that we are still not getting straightforward answers from the NSA," Leahy said in a statement. "I plan to hold another hearing on these matters in the Judiciary Committee and will continue to demand honest and forthright answers from the intelligence community."

Obama has repeatedly said that Congress was thoroughly briefed on the programs revealed by Snowden in June. The two that were described vacuum up vast amounts of metadata -- such as telephone numbers called and called from, the time and duration of calls -- from most Americans' phone records, and scoop up global Internet usage data.

Proposed legislation to dismantle the programs was narrowly defeated last month in the House, and at least 19 other pending bills are aimed at restraining NSA's powers or changing how the agency is regulated, according to a count kept by the ACLU. The July legislative effort brought together Libertarian-leaning conservatives and liberal Democrats who pressed for change against congressional leaders and lawmakers focused on security.

A week ago, Obama sought to soothe concerns by promising to consider reforms to NSA surveillance.

"It's not enough for me to have confidence in these programs," he said at a White House news conference. "The American people have to have confidence in them as well."

He announced changes such as convening an outside advisory panel to review U.S. surveillance powers, although it is unclear how that would differ from the existing U.S. Privacy and Civil Liberties Oversight Board, mandated by Congress to monitor surveillance and constitutional concerns.

Obama also said the NSA would hire a privacy officer -- though the NSA already has a compliance office. None of those measures would seem likely to stop the kind of inadvertent collection of information that was described in the NSA audit.

Most of the infractions revealed late Thursday involve unauthorized surveillance of Americans or foreign intelligence targets in the United States, both of which are restricted by law and executive order, according to the May 3, 2012 audit, and other top-secret documents.

The May audit counted 2,776 incidents in the preceding 12 months of unauthorized collection, storage, access to or distribution of legally protected communications. Most were reported to be unintended, and many involved failures to take sufficient care or violations of standard operating procedure. They ranged from significant violations of law to typographical errors that resulted in unintended interceptions of U.S. emails and telephone calls.

The most serious incidents included a violation of a court order and unauthorized use of data about more than 3,000 Americans and green-card holders.

Sen. Dianne Feinstein, D-Calif., chairwoman of the Senate Intelligence Committee, backed up the administration on its claims, releasing a lengthy statement on Friday afternoon claiming that most the compliance problems at the NSA happen when the agency inadvertently collects records on a non-American who enters the U.S., a point when the NSA is supposed to follow different procedures.

"The majority of these 'compliance incidents' are, therefore, unintentional and do not involve any inappropriate surveillance of Americans," she said. "As I have said previously, the committee has never identified an instance in which the NSA has intentionally abused its authority to conduct surveillance for inappropriate purposes."

Late Friday, the White House issued a statement saying, "the majority of the compliance incidents are unintentional. The documents demonstrate that the NSA is monitoring, detecting, addressing and reporting compliance incidents."

It directed questions to the National Security Council, and NSC spokeswoman Caitlin Hayden directed questions to the NSA.

NSA spokeswoman Vanee Vines said the number of incidents in the first quarter of 2012 was higher than normal, and that the number has ranged from 372 to 1,162 in the past three years, due to factors such as "implementation of new procedures or guidance with respect to our authorities that prompt a spike that requires `fine tuning,' changes to the technology or software in the targeted environment for which we had no prior knowledge, unforeseen shortcomings in our systems, new or expanded access, and `roaming' by foreign targets into the U.S., some of which NSA cannot anticipate in advance but each instance of which is reported as an incident."

"When NSA makes a mistake in carrying out its foreign intelligence mission, the agency reports the issue internally and to federal overseers -- and aggressively gets to the bottom of it," Vines said.

The Associated Press contributed to this report.