A US Senate hearing Wednesday highlighted concern over the growing use of facial recognition technologies, both for law enforcement use and in big social networks like Facebook.

Senator Al Franken, who chaired the judiciary subcommittee hearing, said the expansion of such biometric technologies poses concern over privacy and civil liberties.

Franken said that in law enforcement, the use of facial recognition "will catch criminals," but "could come at a high cost to our civil liberties" if police use the systems to target "innocent civilians" or political rallies.

He also expressed concern that facial recognition used by Facebook to "tag" or identify members could be abused by divulging information that its members do not want made public.

Representatives of the FBI and Facebook testified about their safeguards, and civil liberties advocates and others offered views on the new technology.

Franken said facial recognition "exists right here today" and that "our federal privacy laws are unprepared to deal with this technology."

"Law enforcement doesn't need a warrant to use this technology," Franken added.

On the commercial side, he said, "if a store wants to take a picture and generate a 'faceprint,' they can do it, and they might even be able to sell it to third parties."

Jerome Pender, an FBI deputy assistant director, said the federal law enforcement agency has a database of 12.8 million photos and plans to expand a pilot project to nationwide use by 2014.

But Pender said the agency "is committed to ensuring appropriate privacy protections are in place" for the system. That includes only allow law enforcement to search the data and that only "mugshots" of persons arrested would be included.

But Franken said he was unconvinced: "I'm concerned that it could be used to identify people marching around a courthouse or at a political rally."

Maneesha Mithal of the US Federal Trade Commission said meanwhile that the watchdog agency is studying how to regulate commercial use of facial recognition for firms like Facebook, where she said 2.5 billion photos are uploaded to each month.

She said one way facial recognition is being used in the private sector is for targeted advertising. For example, a digital kiosk can display an ad believed to be relevant to a person's age or gender after an image is scanned by face recognition technology.

Facebook's Rob Sherman said the social network "only uses a person's friends" to allow tagging and encrypts that data so third parties cannot use it.

But Franken criticized Facebook for making the use of the technology "opt out" so that it is used unless the member changes his profile to opt out.

"Facebook users upload 300 million photos to the site each day," he said.

"Faceprints can be happening without a person's consent or knowledge... I think this information is so sensitive I think it's the kind thing users should be able to opt themselves into."

One case evoked at the hearing was a Carnegie-Mellon University professor's study in which pictures were taken from students walking across campus and matched to photos on dating websites which were supposed to be anonymous, all using publicly available data.

Privacy groups said laws need to be updated to offer more protection.

"Businesses should never use facial recognitions techniques to obtain the actual identity of consumers without the consumer's actual knowledge and informed consent," said a statement submitted to the hearing by the Electronic Privacy Information Center.

"Consumers today enjoy enormous freedom and personal safety because they are able to interact with so many merchants, who are essentially strangers, without concern that they will be secretly tracked and profiled."