Robotics experts have asked the United Nations to act to restrict the use of robots in warfare. More than 100 people have signed an open letter warning of the dangers of “lethal autonomous weapons.”

The letter is aimed at those involved in the UN’s “Convention on Certain Conventional Weapons”, an agreement that’s been in place since the late 1990s and bans or limits weapons that “cause unnecessary or unjustifiable suffering to combatants or to affect civilians indiscriminately.” To date it’s mainly covered weapons such as landmines and lasers that could blind people, but the UN recently decided to explore adding autonomous weapons to the list.

Founders and CEOs of dozens of robotics companies have signed the letter, the most notable being Tesla’s Elon Musk. They have urged the UN to do everything it can to prevent an arms race in weaponized robots, as well as offering technical expertise.

Referring to the weapons, the letter warns that:

Once developed, they 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.