WASHINGTON (AFP) - An internal police investigation was under way in St Louis, Missouri on Thursday after a white officer shot and killed an African-American teen not far from where a similar shooting in August led to weeks of unrest.

St Louis police chief Sam Dotson said the off-duty officer was moonlighting as a security guard when he shot and killed the 18-year-old, who they said opened fire first.

Police said the young man killed in the incident was one of a trio of suspects who caught the officer's attention as he drove around a St Louis neighbourhood late on Wednesday in a private security patrol vehicle.

At one point, the officer got out of the car, the youths took off running, and the officer gave chase.

"One of the gentlemen started to approach the officer in an aggressive manner," the police chief told reporters at a news conference early on Thursday.

"The officer was giving them verbal commands, telling them to stop," he said.

Dotson said that the suspect and the police officer, a six-year veteran of the force, got into a scuffle. The teen then took flight again, at which point the officer noticed for the first time that the youth was carrying a handgun.

As he ran away, "the suspect pointed the gun at the officer and fired at least three rounds at the police officer," Dotson said.

"At that point the officer returned fire... as he was being shot at," the police chief said, adding: "The suspect is deceased."

He said police said they recovered a 9mm handgun at the scene, but news reports said relatives of the youth dispute the police account, and insist that the teen was unarmed.

Local media said the officer fired a total of 17 shots, but it was not clear how many of them struck the teen.

The case recalled the Aug 9 flashpoint shooting in the nearby city of Ferguson of another black teen, Michael Brown.

Brown, 18 was shot at least six times by a white police officer, and his body allowed to lie in the street for several hours.

The incident led to weeks of violence in Ferguson, a St Louis suburb of 21,000 with an African-American majority and an overwhelmingly white police department and town government.

Some demonstrators complained that police used undue force against peaceful protests. The incident prompted a nationwide discussion about race and the US criminal justice system.

Dotson said an internal panel would review the circumstances surrounding the shooting "to make a decision about the officer's actions (to determine) if they were appropriate or not in this situation."

The St Louis Post Dispatch newspaper reported that a crowd of about 300 people formed at the scene a short time later, chanting slogans and demanding justice.

The protests became unruly and several police cars were damaged in the unrest, St Louis police said, saying damage included a broken rear window in one police car.

Officials for a time late Wednesday closed streets in the neighborhood and called out a police tactical unit to restore order.