The US government in 2012 searched for the phone records of less than 300 people in a database containing tens of millions of Americans' phone records, intelligence officials have claimed.
"Only a very small fraction of the information acquired under the programme is ever reviewed," intelligence officials said on Sunday in a statement to US Congress.
The release of the figure of fewer than 300 people being targeted is part of a push by officials to allay privacy concerns following recent disclosures of National Security Agency surveillance programmes that collect massive amounts of data in an effort to thwart terrorist attacks, The Washington Post reported.
The "metadata" collected includes phone numbers dialled and length of call but not call content, caller identity or location information.
The statement noted that the government may not query the database for a particular phone number unless it has "reasonable suspicion" that the number is related to a specific foreign terrorist group, the report said.
The phone records programme, highly classified until recently, dates to 2006 and is authorised under a still-secret interpretation of Section 215 of the Patriot Act.
Under that "business records" provision of the Patriot Act, the US government can seek an order from a special surveillance court to direct phone companies to turn over all call detail records on a daily basis as long as they are relevant to a foreign terrorist investigation.
The government "does not indiscriminately sift through the telephony metadata acquired under this programme," the statement said.
"This programme was specifically developed to allow the government to detect communications between terrorists who are operating outside the US but who are communicating with potential operatives inside the US."
The statement repeated officials' assertions that the NSA surveillance programmes have contributed to the disruption of dozens of terrorist plots in the United States and more than 20 countries.
But some lawmakers said they have seen no evidence that the phone records programme has contributed to disrupting so many plots.
"When the intelligence community asks for a programme that touches on the privacy rights of a significant number of Americans, they have an obligation to lay out how it provides unique value in terms of American security," Senator Ron Wyden said. "I don't think that's being done."