The US state of Missouri is bracing for a new wave of racial unrest after another black teenager was killed by an off-duty white cop.

The shooting of Venderrit D. Myers, the third fatal incident of a black male by Missouri police, happened as activists and other protesters prepared for four days of rallies starting on Friday, over the shooting of Michael Brown two months ago.

Police said Myers was shot on Wednesday after he opened fire on a white, off-duty officer, but Myers' parents said he was unarmed.

Some activists and politicians said Myers was targeted because he was black and were asking the Justice Department to investigate the shooting.

"This here was racial profiling turned deadly," said state Sen. Jamilah Nasheed.

Police said Myers was both armed and aggressive, using a stolen 9mm gun to shoot at the officer, however, Syreeta Myers told the AP news agency by phone on Thursday that her son was holding a sandwich, not a gun.

St. Louis Police Chief Sam Dotson said the officer spotted Myers and two other males, and became suspicious when one of them started to run.

When the officer made a U-turn, all three ran and the officer chased them in his car and then on foot, Dotson said.

During the chase, he got into a physical altercation with Myers, who reportedly ran up a hill, turned and fired at the officer, the chief said.

Reminder of Ferguson case

The officer, who was not hurt, shot back and fatally struck Myers. The officer fired off 17 rounds but it's not clear how many bullets hit Myers.

Ballistic evidence shows Myers fired three shots before his gun jammed, Dotson said. Police said they recovered the 9mm gun, which had been reported stolen on September 26.

The 32-year-old officer, who has been with the department for six years, and whose name was not released, is on paid administrative leave pending an investigation.

The Ferguson case was on protesters' minds as they gathered after the latest shooting.

Organisers said the demonstrations include a march on Saturday in the city of St. Louis.

Brown's August 9 death spurred weeks of unrest in Ferguson and the St. Louis region. A state grand jury is deciding whether the officer who shot him, Darren Wilson, will face charges.

In the aftermath of the latest shooting, Dotson said some in the St. Louis crowd shouted obscenities at officers and damaged three police cars, smashing windows and tail lights.

Police said one man was cited for being a felon in possession of a handgun after an officer spotted him putting on a ski mask inside a car, prompting a search.

Online court documents show Myers was free on bond when he was killed. He had been charged with unlawful use of a weapon, a felony, and misdemeanor resisting arrest in June in St. Louis.

Source: Agencies