Elon Musk isn't kidding when he says A.I. could be humanity's largest "existential threat."

In an open letter, the billionaire founder of Tesla and SpaceX joined more than a hundred global technology executives in urging the U.N. to prevent the mass adoption of killer robots. The petition was released on Monday ahead of a U.N. meeting of government experts on autonomous weapons.

"Lethal autonomous weapons threaten to become the third revolution in warfare," the letter states. "Once developed, they will permit armed conflict to be fought at a scale greater than ever, and at timescales faster than humans can comprehend.... We do not have long to act. Once this Pandora's box is opened, it will be hard to close." Other signatories included Mustafa Suleyman, the founder of DeepMind, a U.K. artificial intelligence company acquired by Google.

This isn't the first time Musk has warned about the dangers of a technology he is, in part, responsible for spreading. Earlier this year, he tweeted that A.I. poses "vastly more risk" than the North Korean nuclear regime.

Meanwhile, the entrepreneur is doing his part to promote more benevolent uses of A.I. Earlier this year, he announced that he'd launched a new startup, called Neuralink. The San Francisco company is building devices to connect the human brain with computers, which could potentially repair afflictions such as brain injuries or cancer lesions.

Not everyone in Silicon Valley agrees with him, though. In a recent Facebook Live session, Facebook CEO Mark Zuckerberg derided "the naysayers [trying to] drum out doomsday scenarios," though he stopped short of calling out Musk by name.

Still, Musk sees the dangers of A.I. in no uncertain terms. "We're going to have the choice of either being left behind, [or] being effectively useless," he said in an April interview.