The bulk collection of Americans' phone records by the government exceeds what Congress has allowed, a federal appeals court has said.
Secret National Security Agency (NSA) documents leaked in 2013 to journalists by contractor Edward Snowden revealed that the agency was collecting phone records and digital communications of millions of citizens not suspected of crimes and prompting congressional reform. Snowden remains exiled in Russia.
A three-judge panel of the 2nd US Circuit Court of Appeals in Manhattan permitted the NSA programme to continue temporarily as it exists, and all but pleaded for Congress to better define where the boundaries exist.
"In light of the asserted national security interests at stake, we deem it prudent to pause to allow an opportunity for debate in Congress that may (or may not) profoundly alter the legal landscape," the opinion written by Circuit Judge Gerald Lynch said.
"If Congress decides to authorise the collection of the data desired by the government under conditions identical to those now in place, the programme will continue in the future under that authorisation," the ruling said. "If Congress decides to institute a substantially modified programme, the constitutional issues will certainly differ considerably from those currently raised."
The appeals judges said the issues raised in a lawsuit by the American Civil Liberties Union (ACLU) illustrated the complexity of balancing privacy interests with the nation's security.
A lower court judge in December had thrown out the case, saying the programme was a necessary extension to security measures taken after the September 11 2001 terror attacks.
The appeals court, which heard two hours of arguments by lawyers in December, said the lower court had erred in ruling that the phone records collection programme was authorised in the manner it was being carried out.
During the December arguments, the judges said the case was likely to be decided by the US Supreme Court.
US attorney general Loretta Lynch said the government is reviewing the court's decision.
She added that the June 1 expiration of the Patriot Act provisions provides opportunities to reauthorise the programme "in a way that does preserve its efficacy and protect privacy".
The court's ruling sharpens the focus on the ongoing congressional debate surrounding the programme.
NSA spokesman Edward Price said the agency is evaluating the ruling and added that it is working closely with Congress on reforms.
Jameel Jaffer, ACLU deputy legal director and lead counsel in the case, said the decision "warrants a reconsideration of all of those programmes, and it underscores once again the need for truly systemic reform".
Representative Adam Schiff, the top Democrat on the House intelligence committee, said he hopes the ruling serves as a "catalyst for an end to bulk collection and the beginning of serious reform".