NEW YORK, United States - A Philadelphia police officer has been charged with murder for shooting a 12-year-old boy in the back after chasing the child who had just opened fire on his unmarked vehicle, the city's district attorney said Monday.

Officer Edsaul Mendoza, 26, who was fired from the police force in April, was arrested on Sunday and taken into custody for the death of Thomas Siderio in early March in a neighborhood of the historic northeastern city, located between Washington and New York.

He was charged, among other things, with "first degree murder," Philadelphia District Attorney Larry Krasner said.

During a press conference, authorities detailed the circumstances that led up to the boy's death.

Mendoza, who was in civilian clothing at the time of the shooting, was on a surveillance operation with three other officers in an unmarked car in Philadelphia, which like many other large US cities is undergoing a surge in crime.

Seeing Siderio and another youth aged 17, the police decided to drive around the block.

It was when they turned on their flashing light that the 12-year-old opened fire on their car, according to the authorities.

Mendoza fired back three times at Siderio, and another officer fired one shot. The boy fled. But according to the district attorney, who said he had video footage that he described as "very difficult to watch", the teenager had already dropped his weapon when he was killed.

"When Officer Mendoza fired the third and fatal shot, he knew the 12-year-old, five-foot-tall, 111-pound Thomas Siderio no longer had a gun and no ability to harm him," Krasner said.

"But he fired a shot through his back nonetheless that killed him," he said.

A representative of a police union, quoted by The New York Times, said that Mendoza, "like every other citizen, is entitled to due process, and we are confident that our judicial system will protect this officer's constitutional right to a fair trial."