Two of Silicon Valley's most powerful figures are officially in a war of words.

To recap: During a Facebook Live Q&A session on Sunday, Mark Zuckerberg criticized Elon Musk for saying artificial intelligence is "the biggest threat we face as a civilization."

"I think people who are naysayers and try to drum up these doomsday scenarios--I don't understand it," the Facebook CEO said. "It's really negative, and in some ways I think it's pretty irresponsible."

Now, Musk has fired back. On Tuesday, the Tesla and SpaceX founder tweeted a dig at Zuckerberg.

Musk has extensive experience with A.I. technology through his business ventures: Tesla's self-driving vehicles rely heavily on artificial intelligence, while his startup Neuralink is trying to link people's brains with computers. He has spoken out against A.I. throughout the last several years, sometimes describing apocalyptic scenarios in which even seemingly harmless technology, like a system used to pick strawberries, can have catastrophic outcomes for humans.

"Let's say it ... picks more and more and it is self-improving, so all it really wants to do is pick strawberries," he told Vanity Fair earlier this year. "So then it would have all the world be strawberry fields. Strawberry fields forever."

Musk's warnings about the technology have become more frequent in recent months. Earlier in July at the National Governors Association Summer Meeting, where he made the comment referenced in Zuckerberg's Facebook Live session, he also pushed for tighter regulations. "A.I. is a rare case," he said, "where I think we need to be proactive in regulation [rather] than be reactive."

Zuckerberg evidently subscribes to a different theory. "Technology can always be used for good and bad," he said Sunday, "and you need to be careful about how you build it, and what you build, and how it's going to be used. But people are arguing for slowing down the process of building A.I.--I just find that really questionable."

Facebook has been turning more and more to A.I. recently. Its algorithms can identify objects in photos, for example, and it has trained chatbots to negotiate as well as humans can.

Inc. has reached out to Facebook and Tesla for comment and will update this story if they respond.