Increasingly autonomous, gun-totting robots developed for warfare could easily fall into the hands of terrorists and may one day unleash a robot arms race, a top expert on artificial intelligence told AFP.

"They pose a threat to humanity," said University of Sheffield professor Noel Sharkey ahead of a keynote address Wednesday before Britain's Royal United Services Institute.

Intelligent machines deployed on battlefields around the world -- from mobile grenade launchers to rocket-firing drones -- can already identify and lock onto targets without human help.

There are more than 4,000 US military robots on the ground in Iraq, as well as unmanned aircraft that have clocked hundreds of thousands of flight hours.

The first three armed combat robots fitted with large-caliber machine guns deployed to Iraq last summer, manufactured by US arms maker Foster-Miller, proved so successful that 80 more are on order, said Sharkey.

But up to now, a human hand has always been required to push the button or pull the trigger.

It we are not careful, he said, that could change.

Military leaders "are quite clear that they want autonomous robots as soon as possible, because they are more cost-effective and give a risk-free war," he said.

Several countries, led by the United States, have already invested heavily in robot warriors developed for use on the battlefield.

South Korea and Israel both deploy armed robot border guards, while China, India, Russia and Britain have all increased the use of military robots.