Police in the US city of St Louis fatally shot a black teenager who they say pointed a gun at them on Wednesday and arrested three protesters, recalling racial tensions sparked by the killing of an unarmed African-American teen in nearby Ferguson, Missouri, a year ago.

St. Louis Police Chief Sam Dotson said the shooting took place in the late morning as officers were carrying out a search warrant and two young black men ran out the back door of the targeted house.

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

Reuters video showed dozens of protesters blocking a nearby intersection shortly after the shooting, monitored by police. Three people were arrested for blocking traffic, police said.

Protesters gathered again at the intersection in the early evening. NBC television affiliate KSDK reported that they threw rocks at officers, who responded with what appeared to be tear gas.

Police identified the slain suspect as Mansur Ball-Bey, 18. The second youth fled and was described in his mid- to late teens.

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

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

A 93-year-old member of the Tuskegee Airmen, a black aviation unit from World War Two, was robbed and carjacked in the neighborhood on Sunday. A woman was killed this week in the area.

St. Louis police said the officers involved in the shooting were white, ages 33 and 29, and each with about seven years on the force. They are on administrative leave.

The shooting came 10 days after the St. Louis area was flooded with protesters marking the anniversary of the Aug. 9 police killing of unarmed black teenager Michael Brown by a white police officer.

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

Wednesday's shooting 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. Missouri prosecutors are reviewing the case for possible criminal charges, and Powell's family has filed a wrongful death suit.- Reuters