Senior U.S. military officials are looking into ways to counter the potential risk of enemy forces, including terrorists, acquiring unmanned aircraft to attack U.S. targets.

Discussion of the growing risk has dominated some security forums for more than a year. Brookings Institution analyst John Villasenor warned in a Los Angeles Times article last year the opening of U.S. airspace to authorized drones could also pose risks of terrorist attacks using them.

At the time such threat perceptions were met with derision and discounted as hysterical and improbable.

But Villasenor wrote technology capable of producing a backpack-size drone fitted with a camera and a warhead already exists.

This week the debate moved forward with senior U.S. Defense Department aides reported to be holding talks to focus on developing anti-UAV technology, Defense News and C4ISR Journal reported.

A Counter Terrorism Technical Support Office in the Pentagon held a bidders' conference for classified counter-UAV technology, the news websites reported.

The office is described as a small technology development agency working with the Defense Department's assistant secretary for special operations and low intensity conflict.

"Various U.S. agencies have put some thought into the area from a military perspective, but this solicitation, issued by a division of the office that aims to protect 'high-risk personnel,' reflects a more specific effort," Defense News said.

Countries known to be producing UAVs include China, Iran and Russia. At least 80 nations worldwide are known to own UAVs of different models and capabilities.

Lebanon's Hezbollah is known to possess UAVs, but only the United States and Israel appear to have used drones against enemy targets, Defense News said.

Russian think tanks anticipated early the risk of terrorist drones, as indicated by a published 2004 study by the Center for Arms Control, Energy and Environmental Studies, Moscow.

No incident of UAV employment in a terrorist attack has yet been reported. However, the Russian study said "media reports reveal that terrorists are actively studying this means of delivery."

A Time magazine report in January this year said "drones are no longer the sole domain of the military, and just as with many new technologies, they can easily fall into the wrong hands."

An earlier analysis by U.S. Army Maj. Darin L. Gaub in the Armed Forces Journal began, "Unready to stop UAVs: It's time to get serious about countering unmanned enemy aircraft."

In an interview, Gaub cited two primary methods of countering UAVs, Defense News reported.

"One is the the ability to shoot it down. And the other is using electronic means: to jam it to cause it to crash, or hijack the frequency and take over the controls."

Defense analysts cited in the latest studies say existing defenses against cruise missiles could not effectively counter drones which can have specific characteristics and "intrinsic challenges."

Swedish aerospace manufacturer Saab said this week it is developing unmanned version of its next-generation JAS Gripen-E multi-role fighter. Gripen is one of the key bidders, alongside Boeing and France's Dassault, for a Brazilian program to replace its air force MiGs and other fighter jets with a new generation multi-role fighter jet.

The rise of the UAVs is prompting many air force procurement agencies worldwide to reconsider planned inventories of manned fighter jets and opt for more unmanned battle-ready air attack systems.