Tesla CEO Elon Musk joined more than 100 technology leaders in signing a new letter that urges the United Nations to protect the world from the "dangers" of deadly autonomous weapons.

The letter, released at the opening of the International Joint Conference on Artificial Intelligence in Melbourne, Australia, warns that these weapons "threaten to become the third revolution in warfare."

"Once developed," the letter continues, "they will permit armed conflict to be fought at a scale greater than ever, and at timescales faster than humans can comprehend."

The letter was signed by more than 116 founders of robotics and artificial intelligence companies from 26 countries, according to the Faculty of Engineering at the University of New South Wales in Sydney, Australia. Toby Walsh, a professor on artificial intelligence at UNSW, unveiled the letter. It is also signed by Mustafa Suleyman, who is co-founder of Google's DeepMind AI project.

Musk, a billionare whose other business ventures include SpaceX and Mars colonization, has repeatedly warned about the dangers of AI. In July, he told America's governors that people "should be really concerned" about artificial intelligence, which "is a fundamental risk for human civilization." Earlier this month he tweeted that AI is more dangerous than North Korea.

In December 2016, 123 nations that are part of the U.N. Convention on Conventional Weapons agreed to set up formal talks on the dangers of autonomous weapons. At the time, 19 countries called for a complete ban, and Human Rights Watch cheered the move towards formal talks as "a major step toward negotiations for a ban" on "killer robots."

The new letter warns that autonomous weapons could be used by despots and terrorists alike against "innocent populations," and even weapons held by more responsible powers could be hacked.

"We do not have long to act. Once this Pandora's box is opened, it will be hard to close," the letter says. It beseaches the U.N. "to find a way to protect us all from these dangers."

IJCAI previously sent a letter in 2015 about dangers of autonomous weapons, signed by thousands of researchers in AI and robotics from around the world, which included the endorsements of Musk, British physicist Stephen Hawking and Apple co-founder Steve Wozniak.