SAN FRANCISCO, United States (AFP) — Police in San Francisco have defended their potential use of killer robots, insisting they would be a "last resort" and only for very dangerous situations.

Detectives in the California city, where residents complain of a spike in crime, were granted permission this week to deploy machines capable of lethal force.

City supervisors said if a high-ranking San Francisco Police Department officer gives the green light, armed robots could be sent in to tackle very violent suspects like mass shooters or suicide bombers.

"The use of robots in potentially deadly force situations is a last resort option," San Francisco Police Chief William Scott said Thursday.

"We live in a time when unthinkable mass violence is becoming more commonplace. We need the option to be able to save lives in the event we have that type of tragedy in our city."

SFPD already has a number of robots in its arsenal, which are remotely controlled and used in "bomb situations, hazardous materials incidents and other incidents where officers may need to keep a safe distance before rendering a scene secure," the force said.

The change in the city's rules will mean "robots could be used to deliver an explosive charge to breach a structure containing a violent or armed subject.