WASHINGTON - The Barack Obama administration has declassified a court order authorizing the collection of US phone records, even as it faced new disclosures about the reach of its secret electronic surveillance.

Under mounting pressure from lawmakers, Deputy Attorney General James Cole said the order spells out how the government can use call data obtained from telecom giants such as Verizon.

Cole told senators that the order "provides that the government can search the data only if it has reasonable articulable suspicion that the telephone number being searched is associated with certain terrorist organizations."

Administration officials confirm that an order to compile phone metadata was issued to a subsidiary of Verizon Communications in April, The Washington Post reported. But a footnote clarifies that "telephone metadata does not include the substantive content of any communication... or the name, address, or financial information of a subscriber or customer."

The order was among three secret documents declassified by the office of Director of National Intelligence James Clapper, which said Clapper "determined that the release of these documents is in the public interest."

The move to confront growing opposition to the secret programs came as the administration faced new disclosures from Edward Snowden, the former intelligence contractor at the center of the controversy.

The latest documents obtained by him and published by British daily the Guardian revealed a secret surveillance system known as XKeyscore that allows US intelligence to monitor "nearly everything a typical user does on the Internet."

The National Security Agency (NSA) in a statement late Wednesday refuted as false "the implication" that its information collection is "arbitrary and unconstrained," and said XKeyscore is used as part of the agency's "lawful foreign signals intelligence collection system."

Snowden, who is now stranded in the transit area of a Moscow airport, fled the United States after downloading NSA files that have formed the basis of one bombshell leak after another.

Since he disclosed the vast scale of US electronic surveillance operations in June, public sentiment in the US has shifted against them.

Last week the House of Representatives came within seven votes from defunding the bulk phone records collection program.

At a Senate Judiciary Committee hearing, Cole and US intelligence officers defended the metadata program as a crucial national security tool.

But the panel's chairman, Senator Patrick Leahy, sharply questioned the usefulness of the information gathered.

"If this program is not effective, it must end. And so far, I am not convinced by what I have seen," Leahy said.

Officials said the data has contributed to disruption of 54 terror threats or events, including 13 on US soil.

Deputy FBI director Sean Joyce insisted that the telephone data program, authorized by the Patriot Act that came into law shortly after the 9/11 attacks of 2011, has played a crucial role in "closing the gaps and seams" of intelligence gathering.

"We must have the dots to connect the dots," Joyce said. But Joyce nevertheless could not say that the phone records collection was a singularly critical tool, as had been suggested earlier by intelligence leaders. "Each tool plays a different role," Joyce said. "I'm not saying it's the most important tool."

Leahy said there would always be more "dots" to collect and analyze. "We need straightforward answers, and I'm concerned we're not getting them."

DNI general counsel Robert Litt said the White House was "open to re-evaluating this program in ways that perhaps can provide greater confidence and public trust."

Congress will play a vital role. "There are going to be some proposals to changes in law," Leahy said, citing legislation he and others have introduced to restrict metadata collection to those Americans linked to an ongoing terrorism investigation. But fellow Democrat Dianne Feinstein, chair of the Senate Intelligence Committee, warned against efforts that would hamstring US phone or Internet data collection operations.

"We would place this nation in jeopardy if we eliminate these programs," she said.

Feinstein argued that just 22 vetted NSA analysts were authorized to query the mass of data collected by the programs.

The hearing turned at times to Snowden and how he was able to disappear with classified data. "In this case I think we can say it failed," NSA deputy director John Inglis said of the agency's efforts to prevent leaks.