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Terrorist groups may soon deploy killer robots against unsuspecting civilians, a British researcher warned Wednesday.

Professor Noel Sharkey of the University of Sheffield, in an address given to a defense-oriented think tank in London, pointed out that combat robots, or "autonomous weapons systems," were already being used by several countries.

"The trouble is that we can't really put the genie back in the bottle," he said in a press release before his speech to the Royal United Services Institute. "Once the new weapons are out there, they will be fairly easy to copy. How long is it going to be before the terrorists get in on the act?"

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Sharkey pointed out that the U.S. military already has 4,000 robots on the ground in Iraq, as well as dozens of drone aircraft flying overhead there and in Afghanistan.

Last month, a rocket fired from a drone killed an Al Qaeda leader in Pakistan, and this week, the Pentagon unveiled the Crusher, a nearly unstoppable truck-sized ground-based surveillance robot.

"With the current prices of robot construction falling dramatically and the availability of ready-made components for the amateur market, it wouldn't require a lot of skill to make autonomous robot weapons," said Sharkey.

That may sound farfetched, but in fact iRobot, which makes the Roomba cleaning robots, also makes bomb-defusing and surveillance robots for the Pentagon.

The company encourages reprogramming of the Roomba, and has even created a special version strictly for hackers and tinkerers.

Sharkey doesn't want to get into detail, but says a small drone aircraft with GPS navigation could probably be made for about $500.

• Click here for the University of Sheffield press release.