An international effort from leaders in technology and artificial intelligence (AI) spearheaded by Elon Musk is calling on the United Nations to take proactive measures against the spread of future lethal autonomous technology: killer robots.

The open letter signed by over 100 industry leaders to include the renowned CEO of Tesla and SpaceX cites the threat of “Lethal Autonomous Weapon Systems” as the “third revolution in warfare,” following gunpowder and nuclear weapons, that will “permit armed conflict to be fought at a scale greater than ever, and at timescales faster than humans can comprehend.”

“These can be weapons of terror, weapons that despots and terrorists use against innocent populations, and weapons hacked to behave in undesirable ways.”

The letter ends with an ominous warning: “We do not have long to act. Once this Pandora’s box is opened, it will be hard to close.”