Federal judge William Pauley in New York threw out a petition from the American Civil Liberties Union and said the National Security Agency program was vital in preventing an Al-Qaeda terror attack on American soil.

Ten days earlier, however, another federal judge in Washington declared that this "almost Orwellian" surveillance is probably unconstitutional, laying the groundwork for a protracted legal fight.

"The question for this court is whether the government’s bulk telephony metadata program is lawful. This court finds it is," said the 54-page ruling published in New York on Friday, local time.

Edward Snowden, a former NSA contractor, sent shockwaves around the world this year by revealing the extent of Washington’s electronic eavesdropping on millions of private calls.

The Justice Department welcomed Friday’s ruling but the American Civil Liberties Union said it would appeal.

Pauley said the program, since it is protected by judicial, executive and congressional oversight, does not violate the US constitution’s ban on unreasonable searches and seizures.

"There is no evidence that the government has used any of the bulk telephony metadata it collected for any purpose other than investigating and disrupting terrorist attacks," he wrote.

The judge sided with US spy chiefs who say that by connecting the dots between archived calls and terrorist suspects, US officials can keep the country safe.

The NSA hoovers up information about virtually every telephone call to, from and within the United States, and says it is the only way to discern patterns left behind by foreign terror groups.

The judge quoted the 2004 report by the 9/11 Commission - the panel which investigated the 2001 Al-Qaeda attack on the United States - as saying it was a false choice between liberty and security, as "nothing is more apt to imperil civil liberties than the success of a terrorist attack on American soil."

"As the September 11th attacks demonstrate, the cost of missing such a thread can be horrific. Technology allowed Al-Qaeda to operate decentralised and plot international terrorist attacks remotely," he wrote.

"The bulk telephony metadata collection program represents the government’s counter-punch: connecting fragmented and fleeting communications to reconstruct and eliminate Al-Qaeda’s terror network."

The judge quoted examples in which NSA phone monitoring in 2009 exposed an Al-Qaeda plot to bomb the New York subway, and cite a plot by convicted Pakistani-American terrorist David Headley to bomb a Danish newspaper office.

"Unintentional violations of guidelines," Pauley said, appeared to have stemmed from "human error" and "incredibly complex computer programs" and had been rectified where discovered.

AFP