Police in the US state of Missouri have fatally shot a black teenager, who they say pointed a gun at them, and later faced angry crowds, reigniting racial tensions in the country.

Sam Dotson, the police chief of the city of St Louis, said the shooting took place on Wednesday when young black men ran out of the back door of a house where two officers were carrying out a search warrant.

Officers ordered the pair to stop in an alley behind the house. One suspect pointed a gun at officers who then fired four times, killing him, Dotson said.

"Detectives were looking for guns, looking for violent felons, looking for people that have been committing crimes in the neighbourhood," he said.

Police identified the slain suspect as Mansur Ball-Bey, 18. The second teenager fled.

Both officers, who are white, were unharmed, police said, adding that they were on administrative leave.

Following the incident, crowds gathered at a nearby intersection shortly after the shooting and then again in the evening. Three people were arrested for blocking traffic, police said.

NBC television affiliate KSDK reported that some in the crowd threw rocks at officers, who responded with what appeared to be tear gas.

Unrest after shooting

St. Louis television station Fox 2 showed officers in riot gear lined up across a street, a burning mattress and clouds of smoke or tear gas.

St. Louis Alderman, Antonio French, posted on Twitter that a vacant house and a car were set on fire, and that firefighters were working under heavy police guard.

Dotson the police chief told reporters that Ball-Bey's gun was stolen and said officers recovered crack cocaine at the scene.

The city police said the officers involved in the shooting were white, aged 33 and 29, each with about seven years on the force.

The shooting came 10 days after the city was flooded with protesters marking the anniversary of the killing of unarmed black teenager Michael Brown by a white police officer on August 9 last year in Ferguson, not far from St Louis.

Brown's death helped spark a nationwide movement against what protesters say is police violence against minorities.

Wednesday's shooting also came as activists were in the area to mark the anniversary of the police shooting of another black man in St. Louis, Kajieme Powell. Police say officers shot Powell when he approached them with a knife.

Source: Agencies