The US National Security Agency (NSA) overstepped its legal authority thousands of times each year since 2008, the Washington Post newspaper has reported citing an internal audit.
The NSA audit, dated May 2012 that was provided to the newspaper by Edward Snowden, the former NSA whistleblower, earlier this summer, counted 2,776 incidents in the preceding 12 months of unauthorised collection, storage, access to or distribution of legally protected communications.
Most of the infractions involved unauthorised surveillance of Americans or foreign intelligence targets in the US, which it opted not to disclose to the public or Congress, the paper said on Thursday.
They ranged from significant violations of law to typographical errors that resulted in unintended interception of US emails and telephone calls.
According to the Washington Post, the most serious incidents included a violation of a court order and unauthorised use of data about more than 3,000 Americans and "green card" holders.
In one of the documents, agency personnel are instructed to remove details and substitute more generic language in reports to the Justice Department and the Office of the Director of National Intelligence.
In one instance, the NSA decided it need not report the unintended surveillance of Americans, the US daily said.
A notable example in 2008 was the interception of a "large number" of calls placed from Washington when a programming error confused US area code 202 for 20, the international dialing code for Egypt.
In 2008, the Foreign Intelligence Surveillance Act (FISA) granted NSA broad new powers in exchange for regular audits from the Justice Department and the office of the Director of National Intelligence and periodic reports to Congress and the surveillance court
US District Judge Reggie Walton, who is the chief of Foreign Intelligence Surveillance Court (FISC), told the Washington Post in a written statement that the court does not have the ability to keep the government's surveillance in check because it lacks the tools to independently verify when the NSA has broken its rules.
He also told the newspaper that it cannot verify whether the NSA's employees really did unintentionally make typographical errors.
The NSA said in a statement in response that the agency tries to identify problems "at the earliest possible moment, implement mitigation measures wherever possible, and drive the numbers down", the newspaper reported.
"We're a human-run agency operating in a complex environment with a number of different regulatory regimes, so at times we find ourselves on the wrong side of the line," a senior NSA official told the Washington Post.
Snowden, who revealed details of the secret surveillance programme to the Washington Post and Britain's Guardian, is now in Russia where he has been granted temporary asylum.
He has spoken to his father via the internet for the first time since fleeing the US, media reports said on Friday.
"This decision was taken by Snowden independently, contrary to the recommendation of his attorneys," the Russian lawyer for Snowden said.