US intelligence agencies will stop bulk collection of data documenting calls by US telephone subscribers in June, unless Congress extends a law authorising the spying, US officials said on Monday.

The disclosure that the National Security Agency was collecting metadata generated by domestic telephone users was one of the most controversial revelations made by former NSA contractor Edward Snowden nearly two years ago.

A spokesman for President Barack Obama's National Security Council said abandoning the mass collection of domestic telephone data would deprive the country of a "critical national security tool."

The current law, due to expire on June 1, allows the NSA to collect bulk data on numbers called and the time and length of calls, but not their content.

Efforts by Congress to extend the law so far have proved fruitless, and Congressional aides said that little work on the issue was being done on Capitol Hill.

There are deeply divergent views among the Republicans who control Congress. Some object to bulk data collection as violating individual freedoms, while others consider it a vital tool for preventing terrorist attacks against America.

Ned Price, a national security council spokesman, told Reuters the administration had decided to stop bulk collection of domestic telephone call metadata unless Congress explicitly re-authorises it.

Some legal experts have suggested that even if Congress does not extend the law the administration might be able to convince the secretive Foreign Intelligence Surveillance Court to authorise collection under other legal authorities.

But Price made clear the administration now has no intention of doing so, and that the future of metadata collection after June 1 was up to Congress.

Price said the administration was encouraging Congress to enact legislation in the coming weeks that would allow the collection to continue.

But Price said: "If Section 215 (of the law which covers the collection) sunsets, we will not continue the bulk telephony metadata program."

"Allowing Section 215 to sunset would result in the loss, going forward, of a critical national security tool that is used in a variety of additional contexts that do not involve the collection of bulk data," he said.

Last year the Administration proposed that if collection does continue, the data should be stored by telephone companies rather than NSA itself, but that approach was rejected by the phone companies.

US officials have said metadata collection had helped important counter-terrorism investigations.

However, a review panel appointed by Obama to examine the effectiveness of surveillance techniques revealed by Snowden found that not a single counter-terrorism breakthrough could be attributed to the practice.