Chicago (AFP) - President Barack Obama said he was "deeply disturbed" by video of a white Chicago policeman shooting dead a black teenager, in the latest such incident to roil the United States.

Graphic footage released shortly after officer Jason Van Dyke was charged with first-degree murder on Tuesday has reignited impassioned debate about the use of force by law enforcement in the US, with Chicago left dangerously on edge.

Protesters there have likened the Laquan McDonald killing to that of Michael Brown, the black teenager shot dead by a white policeman in Ferguson, Missouri last year, triggering 15 months of demonstrations in major US cities over perceived police brutality against black men.

A small band of demonstrators hit the chilly Chicago streets for a second night in a row Wednesday, reportedly confronting police officers.

Chicago police initially said that McDonald was high on the hallucinogen PCP, acting erratically and then lunged at officers with a knife when he was shot 16 times in October 2014.

But police dashcam video showed the 17-year-old walking away when Van Dyke opened fire and made no threatening gestures to justify the use of deadly force, prosecutors say.

"Like many Americans, I was deeply disturbed by the footage of the fatal shooting of 17-year-old Laquan McDonald," Obama wrote on his official Facebook page.

"This Thanksgiving, I ask everybody to keep those who've suffered tragic loss in our thoughts and prayers, and to be thankful for the overwhelming majority of men and women in uniform who protect our communities with honor.

"And I'm personally grateful to the people of my hometown for keeping protests peaceful."

Prosecutors and Chicago officials have come under intense criticism for trying to block the release of the video and taking so long to press charges against Van Dyke.

It was the first time a Chicago police officer has been charged with first-degree murder for an on-duty fatality in more than 30 years, the Chicago Tribune reported.

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- Dead on road -

Prosecutors said Van Dyke, who joined the Chicago police in 2001, opened fire just 30 seconds after his vehicle pulled up to the scene with McDonald and six seconds after stepping out of it.

Shot from an approaching police vehicle, the dashcam video shows McDonald run down the middle of the street towards another police cruiser, hitch up his pants and then start to walk away from Van Dyke and his partner.

His body then spins and strikes the pavement. McDonald lifts his head, moves an arm and then a cloud from another gunshot rises up from his chest as he lies in a fetal position.

He does not move as an officer enters the frame for just long enough to kick a knife away from his prone hand.

None of the officers approaching McDonald try to help him as he bleeds out on the street, writhing once in the remaining minute of video.

The fatal shooting put Van Dyke's record in the force under scrutiny, as it emerged that he had at least 20 complaints filed against him but was never disciplined.

That number since 2010 is below the average allegation rate, according to the Citizen Police Data Project from the University of Chicago.

However, he is still among the "small subset" of officers responsible for a "disproportionate" number of complaints.

- 'Rocked to core' -

The latest case has been particularly painful for Edward Nance, who won a $350,000 civil judgment for injuries sustained when he was arrested by Van Dyke and his partner in 2007.

"It rocked him to the core," Nance’s attorney, Michael McCready, told the Chicago Sun-Times.

"He said if they had done something about this cop in our case, this young boy would still be alive."

Van Dyke was a member of the city's controversial and now-disbanded Tactical Response Unit which patrolled high-crime areas when he pulled Nance over because his car didn't have a front license plate.

Nance -- who worked at a cable company and as a high-school sports referee -- told investigators that Van Dyke's partner slammed his head on the hood of his car and that van Dyke violently handcuffed him and tossed him into a squad car.

The Independent Police Review Board dismissed the complaint because there were "no independent witnesses" and "no way to determine" the cause of Nance's injuries, the Sun-Times reported.