A White House-appointed panel has proposed curbs on some key National Security Agency surveillance operations.
In a 300-page report, the five-member panel recommended limits on the agencies programme of collecting records of billions of telephone calls, as well as new tests before Washington spies on foreign leaders.
The panel's report expressed deep scepticism about both the value and effectiveness of the bulk collection of the phone call records, known as "metadata", which the authors recommended should be halted.
Instead, it said, those records should be held by telecommunications providers or a private third party with the US government requiring an order from the Foreign Intelligence Surveillance Court for each search of the data.
"We don't see the need for the government to be retaining that data," said Richard Clarke, a member of the panel and a former White House counterterrorism adviser.
The report's authors say that the metadata collection program "has made only a modest contribution to the nation's security." The program "has generated relevant information in only a small number of cases" that might have led to the prevention of terrorist attack, they said in a footnote.
"The question is not whether granting the government (this) authority makes us incrementally safer, but whether the additional safety is worth the sacrifice in terms of individual privacy, personal liberty and public trust," it said.
It added that "there has been no instance in which the NSA could say with confidence that the outcome would have been different without the... telephony meta-data program. Moreover, now that the existence of the program has been disclosed publicly, we suspect that it is likely to be less useful still."
In another major recommendation, the panel proposed five tests it said should be met before Washington conducts surveillance against foreign leaders.
Revelations in documents provided by Snowden that the United States spied on German Chancellor Angela Merkel and Brazilian President Dilma Rousseff have enraged those countries' citizens.
Before spying on foreign leaders, the panel said, US leaders should determine whether such surveillance is merited by "significant threats" to national security, and whether the nation involved is one "whose leaders we should accord a high degree of respect and deference."
US leaders should also determine whether there is reason to believe the foreign leader has been duplicitous, whether there are other ways to obtain the necessary information, and weigh the negative effects if the surveillance were to become public, the panel said.
White House press secretary Jay Carney said some of the outside panel's 46 recommendations could be accepted, others studied further, and some rejected.
Obama has already rejected, at least for now, one of the panel's proposals: that the NSA and US Cyber Command, which conducts cyber warfare, have separate leaders, with the NSA led by a civilian rather than a military officer.