More than 100 leading artificial intelligence (AI) experts are calling on the United Nations to prevent the development and use of killer robots.

In an open letter to the UN published Monday, a group of 116 robotics specialists – including Tesla‘s Elon Musk and Google’s Mustafa Suleyman – urged the organization to take action against “weapons of terror.”

“Once developed, lethal autonomous weapons will permit armed conflict to be fought at a scale greater than ever, and at timescales faster than humans can comprehend,” the letter said.

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

A UN group had been scheduled to meet on Monday to discuss autonomous weapons, including drones, automated machine guns and tanks. However, the meeting has been postponed until November, according to the group’s website.

AI experts from across 26 different countries emphasized the pressing need for the UN to take action on Monday, while they warned lethal autonomous weapons threatened to become the “third revolution in warfare.”

“We do not have long to act. Once this Pandora’s box is opened, it will be hard to close,” they said.

In 2015, over 1,000 tech experts, researchers and scientists signed a letter warning about the dangers of autonomous weaponry. Among the signatories two years ago were scientist Stephen Hawking, Apple co-founder Steve Wozniak and Elon Musk.