CHICAGO (AFP) - An Ohio police officer who shot a black man during a routine traffic stop was charged with murder on Wednesday in what prosecutors called a "senseless" act motivated by anger.

"He wasn't dealing with someone who was wanted for murder - he was dealing with someone with a missing licence plate," Hamilton County prosecutor Joseph Deters told reporters.

"This is in the vernacular a pretty chicken crap stop. If he started rolling away, seriously, let him go. You don't have to shoot him in the head."

The case comes as the United States grapples with heightened racial tensions in the wake of a series of high-profile incidents of African Americans being killed by police in disputed circumstances.

Deters said he hopes the swift action by his office will show that justice is being done in this case.

"I feel so sorry for his family and I feel sorry for the community," Deters said.

University of Cincinnati campus police officer Ray Tensing initially told investigators that he shot Sam DuBose in the head after DuBose tried to drive away and dragged the officer along with him.

But a review of the officer's body camera footage showed Tensing was never in danger during the July 19 incident.

"You will not believe how quickly he pulls his gun and shoots him in the head. It's maybe a second. It's incredible. And so senseless," Deters said as he prepared to release the video.

"I think he lost his temper because Mr DuBose wouldn't get out of his vehicle."

The video shows Tensing approach the black car and ask DuBose for his licence and registration.

DuBose calmly asks why he was pulled over and eventually tells Tensing that he left his licence at home.

Then - less than two minutes into the exchange - DuBose reaches for the keys and Tensing can be heard shouting "STOP! STOP!"

In the blink of an eye, a gun pops into view and DuBose slumps over in his seat. The video bounces as Tensing chases after the car as it rolls down the street. DuBose died instantly, Deters said.

Tensing should never have been allowed to carry a badge and gun, Deters said, adding that the University of Cincinnati should hand policing duties over to the city's force.

"This is the most asinine act I have ever seen a police officer make," he said.

"It was totally unwarranted and it's an absolute tragedy that in 2015 anyone would behave in this manner."

The university shut down its campus and placed barricades at entrances out of concern that the news could lead to protest or even violence.

City officials pressed for peace and said they were prepared "for any scenarios that present themselves."

A series of sometimes violent protests have broken out across the United States in response to other high-profile police shootings over the past year.

Cincinnati was struck by days of violent unrest following the police shooting of an unarmed black man in 2001.

"There is obviously reason for people to be angry," Mayor John Cranley said.

"Everyone has the right to peacefully protest, but we will not tolerate lawlessness." DuBose's family asked people to respect his memory by responding peacefully as they vowed to continue to fight for justice in policing.

"My brother was about to be just one other stereotype and now that's not going to happen," Terina Allen, DuBose's sister, told reporters.

"I'm as pleased as I can be that we're actually getting some kind of justice for Sam."

Tensing, 25, had been a police officer for four years, the Cincinnati Enquirer reported.