The US National Security Agency's bulk collection of phone records provides only minimal benefits to countering terrorism, is illegal and should end, a federal privacy watchdog has concluded.
The watchdog's report released on Thursday called on the government to end the NSA programme and to purge the data it had collected.
Members of the watchdog questioned the relevance of collecting metadata - records of US phone calls, their length and time - in bulk and its routine reauthorisation by a secretive surveillance court.
The report came on the same day that NSA spying whistle-blower Edward Snowden said he cannot come back from Russia because does not expect to get a fair trial in the US.
The latest report could further complicate efforts by both President Barack Obama and Congress to come up with reforms to the programmes, which have been under harsh scrutiny for their scope in the wake of Snowden's revelations.
The report was a result of a review of the phone data collection programme, based on public and classified briefings and documents, conducted by the Privacy and Civil Liberties Oversight Board at the request of Obama and legislators.
The three-member majority of the board argued that the NSA improperly relies on a statute meant to give the FBI access to phone records.
The board said other existing laws could be used to access phone records if needed.
"It's time to push the reset button" on the programme, said board member James Dempsey, public policy vice president at privacy advocacy group Centre for Democracy and Technology.
Based in part on classified briefings and documents, the board found no terrorist attacks thwarted or previously unknown terrorist plots discovered through the program in the previous seven years.
By concluding that NSA's bulk collection of Americans telephone metadata lacks legal grounding, the board goes further than both President Barack Obama and an ad hoc panel he created to review NSA eavesdropping activities.
Obama, in his own speech on NSA reforms on Friday, did not call for an outright halt to the collection of phone metadata by the NSA, although agreed that the storage of telephone metadata should be moved out of government and should not be searched without judicial approval.
Meanwhile in Russia, Snowden faced an online question-and-answer session on the controversy.
During the session, Snowden said he cannot come back from Russia because "there's no chance to have a fair trial" and urged the US to strengthen its protections for whistle-blowers.
Snowden, living in temporary asylum in Russia after disclosing US government secrets on surveillance programmes and other activities, faces criminal charges in the US States after fleeing last year first to Hong Kong and then Russia, where he was granted at least a year's asylum.
His comments, made on a "Free Snowden" website, came as Eric Holder, US attorney general, said in Virginia that the US government would not consider clemency for him.
"If Mr Snowden wanted to come back to the United States, enter a plea, we would engage with his lawyers. We'd do that with any defendant who wanted to enter a plea of guilty," Holder said.
But Snowden argued that the law under which he was charged "was never intended to be used against people working in the public interest and forbids a public interest defence."
Snowden also said that while he was concerned about this "direct threat to my life", he was "not going to be intimidated".
He also denied that he stole the passwords of NSA co-workers, condemned threats to his life made by unnamed US intelligence officials in the news media, and decried "indiscriminate mass surveillance" by governments.