A US police officer who was shown on video telling a woman during a traffic stop in Georgia not to worry because police "only kill black people" has retired, after authorities sought the officer's dismissal.

Dash cam video showed the Cobb County police officer standing by a car pulled over alongside a road last year and instructing the woman in the front passenger seat to use the mobile phone in her lap to make a call.

"OK I just don't want to put my hands down," the woman tells the officer, adding that she has seen "way too many videos of cops," before trailing off.

"But you're not black," the officer replies. "Remember, we only shoot black people. Yeah. We only kill black people, right? All the videos you've seen, have you seen white people get killed?"

Officials identified the officer as Lieutenant Greg Abbott.

Atlanta TV station WSB-TV, which obtained the video through public records laws, this week broadcast a short excerpt of a longer encounter.

Numerous videos from dash cams, body-worn cameras and mobile phones in recent years have shown US police officers killing unarmed people during encounters, sparking debate about excessive use of force and racial disparities in the criminal justice system.

"This afternoon, after 27 years of faithful service to the citizens of Cobb County, Lt Greg Abbott made the decision to retire," his attorney, Lance LoRusso, said in an email.

Earlier yesterday, Cobb County Police Chief Mike Register said he knows Abbott to be "honourable" but that authorities would begin the process of firing him.

"I don't know what's in his heart but I certainly know what came out of his mouth, and its inexcusable," the police chief said at a news conference.