Killer robots could become the weapon of choice for militants, a British expert said on Wednesday.

Noel Sharkey, professor of artificial intelligence and robotics at the University of Sheffield said he believed falling costs would soon make robots a realistic option for extremist groups.

Several countries and companies are developing the technology for robot weapons, with the U.S. Department of Defense leading the way. More than 4,000 robots are deployed in Iraq.

"The trouble is that we can't really put the genie back in the bottle. Once the new weapons are out there, they will be fairly easy to copy," Sharkey will tell a one-day conference organized by Britain's Royal United Services Institute on Wednesday.

"How long is it going to be before the terrorists get in on the act? 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."

Sharkey said a small GPS-guided drone with autopilot could be made for about 250 pounds ($490).

(Reporting by Ben Hirschler - Reuters)