Lieutenant Greg Abbott’s lawyer said he had made the decision to retire “after 27 years of faithful service” to Cobb County in Georgia, just hours after local police chief Mike Register confirmed that he would be fired for the remark.

The comments were made in July 2016 but revealed on Thursday by WSB-TV, which obtained the video — taken by a dashboard camera — through a freedom of information request.

Mr Abbott told a woman who said she was frightened to lower her hands because she had seen “way too many videos” of police behaviour: “But you’re not black.

“Remember, we only kill black people. Yeah. We only kill black people right?” He then asks the woman if she has seen “the black people get killed” in the videos she referred to.

The woman was a passenger in a car whose driver had been pulled over for dangerous driving.

The driver’s lawyer Suri Chadha Jimenez said the comments were “nasty, sarcastic.

“The reality is to us minorities, there is a real fear when you are pulled over,” he said. “He thought it’s a joke, but it’s not a joke to many people.”

Mr Register said that the officer would have to be fired for the “mistake,” while saying he was an “honourable” man.

But American Civil Liberties Union Georgia executive director Andrea Young said: “Under zero circumstances is it ever appropriate to joke about police officers committing murder.”

She called on the Cobb County police force to undertake diversity training.

Unrest over repeated police killings of unarmed black people led to the creation of the Black Lives Matter movement following the death at police hands of Michael Brown in Ferguson, Missouri, in 2014, after which police attacks on demonstrators caused an international outcry.

But the shootings have continued — with leading Ferguson activist Darren Seals found shot dead late last year — and racial tension has grown since the election of Donald Trump, with clashes between white supremacists and anti-fascists seeing Heather Heyer killed by a far-right thug at Charlottesville last month.