The US National Security Agency would need to begin winding down what it considers its most valuable intelligence program before its expiration at year-end if the US Congress leaves its reauthorisation in limbo, the agency's deputy director said today.

The possibility the US government may begin losing access to the surveillance authority even before it would officially lapse on December 31 is likely to increase pressure on lawmakers to quickly renew the law.

"We would have to be looking to work with our mission partners in the government as well as the companies to start scaling down in advance," George Barnes, the deputy director of the NSA, said at the George Washington University Center for Cyber & Homeland Security event.

"We would, definitely. The last thing we would want to do is conduct any operation ... if we did not have an active statute in place," Barnes said in response to a question asked by Reuters. "We would have to work the dates backwards to make sure we didn't cross the line."

The NSA did not immediately respond when asked about the remarks from Barnes.

The law, Section 702 of the Foreign Intelligence Surveillance Act, allows US intelligence agencies to eavesdrop on, and store vast amounts of, digital communications from foreign suspects living outside of the United States.

It is considered a critical national security tool by US officials, who say it supports priorities ranging from counterterrorism to cybersecurity.

But the program, classified details of which were exposed by 2013 by former NSA contractor Edward Snowden, also incidentally scoops up communications of Americans for a variety of technical reasons, including if they communicate with a foreign target living overseas. Those communications can then be subject to searches from analysts without a warrant.

The scenario articulated by Barnes resembles one that occurred two years ago, when portions of a separate law, the Patriot Act, that allowed the NSA to collect bulk domestic phone metadata were expiring.

Gridlocked over whether to enact reforms, US lawmakers briefly let that Patriot Act lapse. The NSA said it had to begin winding down the program about a week before its expiration.

A bipartisan group of lawmakers in Congress is working on legislation to reform aspects of Section 702, but many Republicans, supported by the White House, want to renew the law without changes and make it permanent.