Dozens of AI-focused technology executives, including Tesla CEO Elon Musk, are urging a United Nations working group to push forward with a plan to ban killer robots.

"Lethal autonomous weapons threaten to become the third revolution in warfare," the group of CEOs and CTOs wrote in an open letter organized by the Future of Life Institute and released Sunday. "Once developed, they will permit armed conflict to be fought at a scale greater than ever, and at timescales faster than humans can comprehend. These can be weapons of terror, weapons that despots and terrorists use against innocent populations, and weapons hacked to behave in undesirable ways."

It's easy to imagine the potential nightmare scenarios here. You only have to think about the Terminator's Skynet, or that Star Trek: The Next Generation episode where Enterprise crew members are hunted down by an automated weapons system that turns out to be a sales demo by a long-defunct arms merchant.

But drafting a workable ban and getting dozens of countries to sign onto it might not be easy.

Countries around the world are actively working on ever-more-advanced autonomous weapons systems. The United States has been using remotely piloted drones in the Middle East for close to a decade, and Israel operates remotely operated gun turrets to guard border fences near Gaza.

The next step, under active development in the United States and elsewhere, will be semi-autonomous weapons systems where human beings only make higher-level decisions—especially about whether to use lethal force—while vehicles make routine navigational decisions on their own. The US military has dubbed this its "centaur" strategy, in which weapon systems take advantage of the best capabilities of both human and artificial intelligence. In this kind of system, human operators might manage swarms of small aircraft or convoys of vehicles, choosing destinations and targets with the click of a mouse from thousands of miles away and letting software figure out specific tactical decisions.

And this points to the reason getting a workable ban might be so challenging: there's no clear line between remotely piloted and fully autonomous systems. Even if humans remain in charge, there are going to be big advantages to programming computers to understand the environment around them, allowing them to navigate through space, identify potential targets, and so forth.

And once drones have the capability to autonomously identify potential threats, the decision of whether to seek human approval before firing on targets will be a question of how robots are configured—something that could easily be changed in the middle of a war if human decision-making proves to be a bottleneck.

So even if countries reached a consensus that a ban on fully autonomous weapons is desirable, it's not clear that a ban would have teeth. Major militaries will have little choice but to develop the technical capabilities to build fully autonomous robots and to quickly convert their semi-automated drones to fully autonomous mode if military necessity requires it. And if that option exists, some country is going to use it.

The threat of lethal robots has been a long-time concern for Musk. He signed a similar letter in 2015 and donated $10 million to the Future of Life Institute to advocate AI technologies that help humanity rather than harm it.