GENEVA, May 23 (Reuters) - A United Nations human rights investigator called on Thursday for all states to declare a moratorium to prevent so-called "killer robots" being deployed on the battlefield.

Christof Heyns, U.N. special rapporteur on executions, said that unmanned robotic weapons systems with varying degrees of autonomy and deadliness were being tested or used by the United States, Britain and Israel without debate on moraI and legal issues.

"Moratoria are needed to prevent steps from being taken that may be difficult to reverse later," Heyns said in a 22-page report on "lethal autonomous robotics", due to be discussed at the U.N. Human Rights Council in Geneva on May 29.

"Their deployment may be unacceptable because no adequate system of legal accountability can be devised and because robots should not have the power of life and death over human beings."

Remotely-piloted drone aircraft, increasingly used to carry out targeted killings in U.S. counterterrorism operations, are problematic in their own right, said Heyns, a South African law professor serving in the independent expert post.

But adding robots to arsenals would bring a new dimension as they would take targeting decisions themselves, which could "open an even larger Pandora's box", he warned.

Technology is developing at an "exponential rate", but programmes are shrouded in secrecy, making it impossible to know how soon fully autonomous weapons might be ready, he said.

Northrop Grumman and Samsung Techwin are among companies with robotic systems being used or tested, Heyns said. Samsung security robots deployed in the demilitarised zone between North and South Korea are operated by humans but have an automatic mode, he said.

ROBOTS FIGHTING HUMANS

Based on current experience with drones - used by the U.S. military in hotspots including Pakistan and Yemen - "there is reason to believe that states will, inter alia, seek to use lethal autonomous robotics for targeted killing", Heyns said.

He welcomed a Pentagon directive last November recognising the need for domestic control of production and deployment of lethal autonomous robotics, "imposing a form of moratorium".

Killer robots might be "pitted against people on foot", he said, while dismissing backers who say that they offer the prospect of "riskless war" and "wars without casualties".

Referring to international humanitarian law that lays down the rules of war, he questioned whether robots would be capable of complying with its requirements, including the need to distinguish between civilians and combatants.

"It would be difficult for robots to establish, for example, whether someone is wounded and hors de combat, and also whether soldiers are in the process of surrendering," Heyns said.

"A further concern relates to the ability of robots to distinguish legal from illegal orders", he added.

Campaigners including Nobel Laureate Jody Williams called in April for a ban on machines with the ability to attack targets without any human intervention.

The U.S. Navy made aviation history on May 14 by launching an unmanned jet - the X-47B stealth drone by Northrop Grumman - from an aircraft carrier for the first time, taking an important step toward expanded use of drones.

Human Rights Watch, in its campaign against killer robots, cited the X-47B as one of several weapons that represent a transition toward development of fully autonomous arms.