LONDON -- Military experts have warned that terrorists could use unmanned drones in aerial attacks, saying robotics offered a frighteningly easy way to evade security.

The know-how and materials for manufacturing lethal, improvised robots are easily available, according to experts at a conference Wednesday on robotics at the Royal United Services Institute, a 177-year-old forum on military affairs.

"Sooner or later we're going to see a Cessna programmed to fly into a building," said Rear Adm. Chris Parry, who formed the Ministry of Defense's Development, Concepts and Doctrine Center in 2005. He said small, remotely piloted planes or even converted model aircraft were "ideal weapons" for terrorists because they are easy to build and could evade radar.

"They are cheap. They don't need as much motor power or fuel, and they're difficult to detect -- about as difficult to detect as a blackbird," he said.

Parry's statements were echoed by other speakers, among them computer scientist Richard Starkey.

"It is very easy to go to the Internet ... or go down to the scrapyard and put a robot together," Starkey said. "You don't need (it) to last long if you want to explode it among a civilian population."

Both pointed to Hezbollah's deployment of pilotless aircraft against Israel in 2006, when the militant group sent a series of unmanned aerial vehicles hovering above Israeli territory. Parry alluded to the use of unmanned submarine-like vessels to ferry drugs across the Pacific.

In February 2003, six Hamas militants died in an explosion as they were examining a remote-controlled model airplane that Israel and Hamas said was intended to be used in an attack.

Al-Qaida-linked groups have also reportedly considered using unmanned aircraft -- in 2006 American radical-turned-FBI informant Mohammed Junaid Babar accused an alleged Canadian co-conspirator, Momin Khawaja, of working on fitting a model plane with explosives.

Unmanned vehicles, from hunter-killer planes like the U.S. Predator to explosives-disposal buggies, are also playing an increasingly important role in the U.S. war effort in Iraq and elsewhere.

The Pentagon wants $3.4 billion for 2008 to fund its unmanned aircraft programs, and a strategy document put out by the U.S. Department of Defense last year outlined plans to automate a third of the Army's new ground combat vehicles by 2015.