US officials have fired six police officers who were involved in a 137-shot barrage that killed two unarmed African Americans in 2012.

The ruling was made on Tuesday, three years after 13 officers fired a total of 137 times at the car of two black Americans, Timothy Russell and Malissa Williams, after a high-speed chase in Cleveland, Ohio.

The car, driven by Russell, backfired with a noise that officers mistook for gunshots, prompting the chase, according to prosecutors. More than 60 police cars pursued the vehicle for 20 miles in a 25-minute chase spanning three cities.

When police finally checked the car, they realized neither Williams nor Russell was armed.

Michael Brelo, a patrolman, was charged because prosecutors said he fired 15 rounds even when the vehicle had stopped and the occupants were no longer a threat.

Officer Michael Brelo adjusts his tie before court proceedings in Cleveland on December 17, 2014.

Standing on the hood of the car, Brelo shot down through the windshield, yet he was acquitted of manslaughter charges in May 2015.

The high-speed chase involved 62 police cruisers and more than 100 officers. The shooting killed both occupants of the car. Timothy Russell was hit by 24 shots, Malissa Williams by 23.

"The politics in this city is absolutely appalling, and those fired will get their jobs back," said Steve Loomis, Cleveland's police union president.

"How many people have to tell us this was a justified shooting? It's tragic that it went down this way, but at the end of the day, two people high on crack cocaine, high on marijuana, one of them intoxicated, made the decisions that they made and we responded," he added.

The fatal shooting preceded mass demonstrations over unconstitutional policing and excessive use of force by police officers.

Cleveland and other US cities, including Ferguson, New York and Baltimore, have been the scene of massive protests over police brutality against African-Americans.

Dozens of protesters were arrested during protests against the acquittal of Brelo and were arraigned in court in Cleveland.

Police brutality in the United States has raised nationwide debates amid a string of police killings of unarmed black men that has led to the creation of the Black Lives Matter movement.