Thousands of tiny unmanned aircraft or drones flying into civilian airspace over the United States can pose a security threat as they may be difficult to monitor in the long run and some craft may fall into enemy hands, security analysts say.

Although debate over the use of surveillance drones, approved by Congress this week, centers on civil liberties and individuals' rights, a much greater risk of hostile drones entering U.S. airspace undetected isn't being considered, analysts said.

The Federation Aviation Administration said up to 30,000 drones could be in airspace shared with airliners carrying passengers.

Current lobbying for the drones insists there will be enough qualified experts to operate the drones safely and not endanger airborne human traffic but there are many questions unanswered about how the drone operators would be regulated.

More important, at a time when government defense cutbacks are the norm, little consideration is being given to potentially unfathomable costs of maintaining a vast fleet of drones, their monitors and operators and the whole regulatory framework required to run the system efficiently and safely.

There is risk, too, that terrorists will attempt to penetrate the drone network with unpredictable consequences for the safety of the set-up as well as citizens, analysts said.

Once the bill has been signed by U.S. President Barack Obama, the FAA Reauthorization Act will allow the FAA to give drone traffic the go-ahead and develop regulations for testing and licensing by 2015.

The expectations are that the law eventually will streamline processes for multilevel licensing of drone flights by federal, state and local police and other government agencies.

The legislation follows vigorous campaigning by defense and security industries that see drones as a multibillion-dollar growth area.

The defense and security industries have already made up for declining revenues in direct defense acquisitions by developing new business to counter cyberthreats, border body scanners and giant merchandise scanners and a range of software to counter Internet fraud, identity fraud called phishing and hacking of sensitive corporate and government data.

The American Civil Liberties Union said the legislation could severely undermine Americans' privacy. The Electronic Frontier Foundation echoed those privacy concerns.

U.S. defense and security forces deployed drones of varying sizes in Afghanistan and Iraq. The CIA's armed Predator drone program targeted al-Qaida leaders but officials said smaller drones were deployed outside U.S. diplomatic areas in Iraq to monitor the safety of U.S. officials during their movements within the country.

In addition to the drones, the race is on to develop unmanned aircraft that can safely share airspace reserved for civilian aviation.

There's both military and commercial interest in having unmanned aircraft that can fly unaided by human pilots, most of the time.

Initially, unmanned aircraft likely to be released for sharing airspace with civilian airliners will have the option to require a pilot. Eventually, however, they could go unmanned and fly into airspace used by manned airliners using devices on board and control centers on the ground, industry data indicated.

The idea of civilian aircraft flying virtually at the mercy of unmanned craft cruising in their midst has delayed commissioning of such craft, both for practical and psychological reasons. But support for unmanned craft joining civil aviation is catching on, reports indicated.

Several companies are deep into research and development of optionally piloted aircraft. Among these, Aurora Flight Sciences of Manassas, Va., is experimenting general aviation Diamond DA42 planes to be able to market it as intelligence, search and reconnaissance planes.

Mav6 Blue Devil 2 airship has been designed to run pilotless or to have a pilot on board when required. The aerospace company, which has headquarters in Vicksburg, Miss., has designed the Blue Devil 2 airship to accommodate a pilot when necessary.

The 370-foot airship can hover at 20,000 feet for five days and function as a network hub for drones and ground sensors and as a surveillance craft.

Northrop Grumman used a pilot to show off its Firebird last year but the company says the plane can fly just as well without a pilot.

Industry analysts said the research and development drive toward pilotless planes was rooted both in defense priorities to deploy an optionally piloted aircraft in difficult conditions and in defense needs to find new uses for unmanned aircraft that are likely to be made idle with the pullout from Afghanistan.

Hundreds of Predator class unmanned aircraft and some high-flying Global Hawks planes are likely to become available with the drawdown in Afghanistan.