A viral video posted on social media shows a white police officer using a stun gun on an unarmed seated black man, despite appearing to follow the officer's orders.

Just  over a week after an East Pittsburgh police officer shot and killed an unarmed black teenager in the back a cop  in another Pennsylvania city used a Taser on the  27-year-old back of a black

That encounter in Lancaster widely shared after being posted to Facebook, quickly led to a public outcry, including from the town’s mayor, who said an investigation would be conducted.

A police officer has been charged with homicide over the shooting of an unarmed black teenager last week, the latest in a string of US police killings of black men that have sparked nationwide protests.

The officer, 30-year-old Michael Rosfeld, surrendered to face a single criminal count for the 19 June death of 17-year-old Antwon Rose, which occurred as the officer was searching for suspects after a drive-by shooting that wounded one.

Video of the incident showed two men running away from a car that had been stopped by police, and falling to the ground amid the sound of gunfire. The video sparked outrage and several nights of protests in the western Pennsylvania city.

he United States has seen repeated waves of protests following police killings of black men in cities including Ferguson, Missouri; Baltimore and North Charleston, South Carolina. The killings fuelled a national debate about racial bias in the US criminal justice system.

Few of the officers who have faced charges for the killings have been convicted, as US laws give police officers broad powers to use deadly force when they believe their lives or the lives of others are in danger.