Washington - A watchdog panel concluded on Thursday that bulk data collection by the United States is illegal and should be stopped, prompting praise from intelligence leaker Edward Snowden.
But the whistleblower said in an internet chat that he has no plans to return home because he would not get a fair trial.
The 30-year-old former government contractor is wanted by Washington on espionage charges for leaking classified documents from the National Security Agency and exposing the vast scope of US spying.
US Attorney General Eric Holder, meanwhile, said he was not prepared to talk about clemency.
American authorities "would engage in conversation" if Snowden accepted responsibility for leaking government secrets but granting clemency "would be going too far", Holder told MSNBC television.
A report released by the privacy and civil liberties oversight board concluded the NSA's huge phone metadata programme is illegal in several ways, and provides little or no value to the fight against terrorism.
The 238-page report said the programme "has shown minimal value in safeguarding the nation from terrorism".
And the panel said the programme is not authorised by the Patriot Act, the law passed following the attacks of September 11.
It said it violates constitutional guarantees of free speech and protection against unreasonable searches, and also fails to comply with a federal privacy law.
Moreover, it said the programme threatens to have "debilitating consequences for journalism" because "sources in a position to offer crucial information about newsworthy topics may remain silent out of fear that their telephone records could be used to trace their contacts".
The report also said the NSA stretches the interpretation of what may be "relevant" to a terrorism investigation.
The board, which was set up to create safeguards for privacy and civil liberties for stepped-up anti-terrorism efforts, rejected the notion the NSA programme could have prevented the 9/11 attacks.
"The programme supplied no advance notice of attempted attacks on the New York City subway, the failed Christmas Day airliner bombing, or the failed Times Square car bombing", the report said.
But the five-member panel, which began work in 2012, years after Congress called for its creation, was split in its conclusions. Two members said they disagreed with key conclusions on the NSA programme's legality and effectiveness.
White House spokesperson Jay Carney said the administration sees the programme as "lawful", but that President Barack Obama has already outlined reforms.
Civil liberties groups, meanwhile, hailed the report.
Jameel Jaffer of the American Civil Liberties Union said the panel correctly concluded that the NSA's call-records dragnet "is illegal and ineffective and presents a serious threat to civil liberties".
Zeke Johnson of Amnesty International USA said the report "should be the final nail in the coffin for the bulk collection of US telephony metadata under Section 215 of the Patriot Act".
"Congress should move to end the programme", Johnson said.
The report comes a month after a special panel named by Obama urged scaling back of electronic spying powers to protect privacy rights and shore up public trust.
The NSA programmes have been in focus since details leaked by Snowden were published in news reports, outraging many activists at home, as well as US allies caught up the data sweep.
'No chance' for fair trial
In a chat organised by the "Free Snowden" website, the former NSA contractor defended his actions, while ruling out plans to return home.
"The hundred-year old law under which I've been charged forbids a public interest defence", he said in a question-and-answer session.
"This is especially frustrating, because it means there's no chance to have a fair trial, and no way I can come home and make my case to a jury."
Snowden, who has been granted temporary asylum in Russia, said the report supports his contention that the NSA programmes violate US laws and the constitution.
He added that the NSA is "setting a precedent that immunises the government of every two-bit dictator to perform the same kind of indiscriminate, dragnet surveillance of entire populations that the NSA is doing".
"It's not good for our country, it's not good for the world, and I wasn't going to stand by and watch it happen, no matter how much it cost me."
Snowden said he tried to alert his superiors about the NSA programmes before he went to the media.
But he said that, as a contractor, he did not have "whistleblower protection" and that his protests did nothing to spur action.
"I still made tremendous efforts to report these programmes to co-workers, supervisors, and anyone with the proper clearance who would listen", he said.
"The reactions of those I told about the scale of the constitutional violations ranged from deeply concerned to appalled, but no one was willing to risk their jobs, families, and possibly even freedom."