Physicist Stephen Hawking and Tesla Motors CEO Elon Musk have joined scores of artificial intelligence and robotics experts calling for a ban on “offensive autonomous weapons” in an open letter published Monday.

“AI technology has reached a point where the deployment of such systems is–practically if not legally–feasible within years, not decades, and the stakes are high,” reads the letter, which the Guardian reports will be presented Wednesday at the International Joint Conference on Artificial Intelligence in Buenos Aires. “Autonomous weapons have been described as the third revolution in warfare, after gunpowder and nuclear arms.”

The letter distinguishes AI weapons, which can select and attack targets without human orders, from drones and cruise missiles whose targets are selected by humans. The letter also says that while artificial intelligence can make war zones safer for members of the military, weapons that can operate without human control would kick off “a global AI arms race.”

“It will only be a matter of time until they appear on the black market and in the hands of terrorists, dictators,” and other bad actors, the signatories warn. “There are many ways in which AI can make battlefields safer for humans, especially civilians, without creating new tools for killing people.”

Other signees include Apple co-founder Steve Wozniak as well as DeepMind founder and Google executive Demis Hassabis.

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Write to Nolan Feeney at nolan.feeney@time.com.