If an out-of-control future AI deems that humans are obstructing its goals, it could use robots to "synthesise and spread novel pathogens", said Stuart Russell, computer science professor at the University of California, Berkeley.

Or it could "convince people to initiate nuclear war, including by hacking into early warning systems", Russell, a prominent voice on AI safety, told AFP.

Such existential fears do not concern current AI systems, he stressed. But as the technology improves it could take lethal action beyond our understanding, such as by removing oxygen from the atmosphere.

"We are less intelligent than superintelligent AI. If you ask chimpanzees how humans could wipe them out, they probably wouldn't be able to give you all the right answers."