Authorities in Chicago have released police body camera footage showing an officer shooting dead a 13-year-old Latino boy as the mayor of the US's third-largest city appealed for calm over the "excruciating" video.

The footage shows Adam Toledo running from officers in the early hours of 29 March and then being hit with a single shot to the chest as he stops and raises his hands.

Prosecutors say he was armed, although no weapon is visible in his hands in the video when he is struck.

Tensions over racism and policing are already high in the United States as nearby Minneapolis hosts the trial of a white former policeman charged with murdering African American George Floyd.

It is taking place in the same week that a fatal police shooting of an unarmed motorist sparked violent protests in a suburb of the city.

In a news conference ahead of the latest video's release, Chicago Mayor Lori Lightfoot called for a peaceful response from residents, describing the footage as "excruciating to watch".

Small groups gathered in downtown Chicago for peaceful protests over the boy's death after the footage was published.

"Lock the cop up!" some chanted, according to local media.

Ms Lightfoot would not comment on whether Adam was holding a gun when he was killed, despite repeatedly saying earlier in the week that he was.

"I don't think it matters whether Adam is a choir boy, whether he is involved in some other untoward activity -- the fact of the matter is that he was walking in the street and he was shot unarmed," Adeena Weiss-Ortiz, the Toledo family attorney, told reporters.

Adam, who reportedly has American Samoan heritage, was buried last week.

The Civilian Office of Police Accountability is reviewing footage from body cameras worn by the responding officers and other video, along with audio recordings from surveillance sensors.

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Police responded to a report of shots fired at 2.30am on the Midwestern city's predominantly Latino and black west side. They chased Adam and 21-year-old Ruben Roman - who had fired the originally reported shots, according to prosecutors - on foot.

Mr Roman was quickly apprehended as the pair fled down an alley and the bodycam footage shows an officer cornering Adam. Prosecutors say the boy was armed, although no gun can be seen in the footage as he
raises his hands.

"The officer tells (Adam) to drop it... as (Adam) turns toward the officer," Cook County assistant state's attorney James Murphy said on Saturday in a bond hearing for Mr Roman.

"(Adam) has a gun in his right hand. The officer fires one shot at (Adam), striking him in the chest," Mr Murphy said.

The gun Adam was allegedly holding, a Ruger 9mm, landed against a nearby fence, Mr Murphy said, and the officer immediately rendered aid - but Adam was pronounced dead at the scene.

There was gunshot residue in the youngster's right hand, and shell casings that were found where Mr Roman had allegedly been firing the weapon matched the Ruger, Mr Murphy added.

Prosecutors have not indicated the point at which they say the gun ended up with Adam.

Yesterday, Chicago network WGNTV reported that the Cook County state's attorney's office said prosecutor Murphy was in fact "not fully informed."

"An attorney who works in this office failed to fully inform himself before speaking in court," Sarah Sinovic, a spokeswoman for Cook County State's Attorney Kim Foxx, told WGN Investigates.