City supervisors last week said police would be allowed to use robots capable of delivering lethal force to deal with dangerous felons and life-threatening situations.
Police had welcomed the plan, stressing that it would be an option of last resort for tackling violent suspects such as mass shooters or suicide bombers, without risk to officers' lives.
But at a meeting of the San Francisco Board of Supervisors on Tuesday that had been set to approve the plan, officials said they now had reservations.
“Last week, despite some ethical concerns, I did support the legislation but over the past week I feel extremely uncomfortable about it,” board member Gordon Mar said.
“I do not think armed and remote robots will make us safer.”
Last week's announcement sparked headlines around the world, with some making comparisons with dystopian sci-fi such as Terminator and Black Mirror.
Protesters attended Tuesday's meeting and had descended on city hall demanding the plan be reversed.
After discussions, supervisors said the decision on killer robots needed to go back to committee for further consideration, although gave the green light to plans to allow police to acquire some military-style equipment.