A U.S. judge has ruled the National Security Agency's mass collection of telephone data unconstitutional.

According to BBC News, Federal District Judge Richard Leon said the electronic spy agency's practice was an "arbitrary invasion".

The agency's collection of "metadata" including telephone numbers and times and dates of calls was exposed by ex-NSA contractor Edward Snowden.

In his ruling in a Washington DC federal court on Monday, Dec 16, Leon called the NSA's surveillance program "indiscriminate" and an "almost Orwellian technology that enables the government to store and analyze the phone metadata of every telephone user in the United States".

The ruling came in a lawsuit brought by conservative activist Larry Klayman, a user of a Verizon mobile telephone who challenged the NSA's collection of metadata on his behalf and that of a client.

The NSA had ordered Verizon - one of the largest phone companies in the U.S. - to disclose to it metadata, including telephone numbers, calling card numbers and the serial numbers of phones, of millions of calls it processes in which at least one party is in the U.S.

Leon ruled the plaintiffs had demonstrated "a substantial likelihood of success on the merits of their Fourth Amendment claim and that they will suffer irreparable harm absent… relief", referring to the clause in the U.S. constitution that bars unreasonable search and seizure by the government.

He issued a preliminary injunction against the NSA surveillance program but suspended the order to allow for an appeal by the justice department, thus enabling the program to continue for now.

Earlier on Monday, the White House rejected the suggestion that Snowden be granted amnesty, a day after a top NSA official publicly suggested a deal could be reached to keep Snowden from leaking more documents, according to the BBC.

White House spokesman Jay Carney said the U.S. government continued to press Russia - where Snowden has been granted asylum - to return him to the U.S.

"There's been no change in our position," he told reporters. "He faces felony charges here, he ought to be returned to the United States, again, where he will face full due process and protection under our system of justice, that we hope he will avail himself of."

On Sunday, Richard Ledgett, head of the NSA's task force investigating damage from Mr Snowden's leaks, discussed the possibility of an amnesty deal on the U.S. television channel CBS.

"My personal view is, yes it's worth having a conversation about," he said. "I would need assurances that the remainder of the data could be secured, and my bar for those assurances would be very high, would be more than just an assertion on his part."

Carney said on Monday that the proposal represented Ledgett's "personal opinion" and such decisions were ultimately made by the Department of Justice.

Earlier, NSA Director Gen Keith Alexander also dismissed the idea."This is analogous to a hostage taker taking 50 people hostage, shooting 10, and then say, 'if you give me full amnesty, I'll let the other 40 go'. What do you do?"