The United Nations has been warned that its protracted negotiations over the future of lethal autonomous weapons – or "killer robots" – are moving too slowly to stop robot wars becoming a reality.

Lobbying for a pre-emptive ban on the weapons is intensifying at the UN general assembly in New York, but a deal may not emerge quickly enough to prevent devices from being deployed, experts say.

"There is indeed a danger now that [the process] may get stuck," said Christof Heyns, the UN special rapporteur on extrajudicial, summary or arbitrary executions.

"A lot of money is going into development and people will want a return on their investment," he said. "If there is not a pre-emptive ban on the high-level autonomous weapons then once the genie is out of the bottle it will be extremely difficult to get it back in."