FERGUSON (AFP/REUTERS) - The family of slain black US teen Michael Brown said they were "profoundly disappointed" by a grand jury's decision not to charge the white police officer who killed him, but called for calm.

"We are profoundly disappointed that the killer of our child will not face the consequence of his actions," the Brown family said in a statement.

"While we understand that many others share our pain, we ask that you channel your frustration in ways that will make a positive change."

Missouri prosecutor Bob McCulloch acknowledged on Monday that the grand jury decision would not be accepted by some people.

But he said it was a good decision to take the case to the grand jury to decide whether any charges were warranted in the Aug 9 shooting death of Michael Brown, and added that the grand jury's members had "poured their hearts and souls into this."

He added that the federal investigation into the shooting is still ongoing.

The death in August of 18-year-old Michael Brown sparked several weeks of rioting in Ferguson, a suburb of St Louis, but on Monday a grand jury decided that Officer Darren Wilson had acted lawfully.

Ferguson police have fired tear gas to disperse protesters on Monday after a white policeman escaped charges for shooting dead a black teenager, even as US President Barack Obama has urged police to show "restraint".

Gunshots were heard and bottles were thrown as anger rippled through a crowd outside the Ferguson Police Department in suburban St Louis after authorities on Monday announced that a grand jury voted not to indict the officer, Darren Wilson, in the August shooting death of the unarmed 18-year-old Michael Brown.

Whistles pierced the air as some of the hundreds of protesters tried to keep the peace, shouting, "Don't run, don't run."

Police who formed a wall of clear riot shields outside the precinct were pelted with bottles and cans as the crowd surged up and down the street immediately after the grand jury verdict was delivered.

"Murderers, you're nothing but murderers," the crowd shouted. One woman, speaking through a megaphone said, "Stinking murderers."

Not far from the stretch of Ferguson that saw the worst of the rioting after Brown's fatal shooting in August, dozens of police and military vehicles were poised for possible mass arrests.

"They need to feel the pain these mothers feel at the (expletive) cemetery," shouted Paulette Wilkes, 40, a teacher's assistant who was in the crowd at the police department.

Mr Obama urged calm on Monday in a press conference held shortly after the grand jury verdict.

"I join Michael (Brown)'s parents in asking anyone who protests this decision to do so peacefully," Mr Obama told reporters. "I also appeal to the law enforcement officials in
Ferguson and the region to show care and restraint in managing peaceful protests that may occur."

The grand jury found there was no probable cause to charge Ferguson, Missouri, police officer Darren Wilson, who is white, with any crime for the shooting of 18-year-old Michael Brown, an incident that highlighted longstanding racial tensions in the predominantly black city, which has a white-dominated power structure.

"They determined that no probable cause exists to file any charge against officer Wilson," St Louis County Prosecutor Bob McCulloch told reporters in Clayton, Missouri, a suburb of St Louis where the grand jury met.

A crowd of several hundred people gathered outside the Ferguson Police Department ahead of the news, and many began to scream angrily as the news of the grand jury's decision was read.

The authorities had stepped up security in and around Ferguson ahead of the decision, and Missouri Governor Jay Nixon called up the National Guard to guard against the kind of rioting that flared in the weeks after the Aug 9 shooting.

Ahead of the decision, officials called on area residents to remain calm following the grand jury's decision.

"This is not the time to turn on each other," St Louis County Executive Charlie Dooley told a news conference. Dooley said he did not want residents to feel they need to barricade themselves in their homes.

"I do not want people to accidentally shoot or harm someone out of fear," Dooley said.

The grand jury, with nine white and three black members, began meeting in late August and heard evidence that included witnesses called by the prosecution as well as a private pathologist hired by the Brown family to review the shooting.

Nine jurors needed to agree to bring charges.

Lawyers for Brown's family say the teen was trying to surrender when he was shot, while Wilson's supporters say he feared for his life and opened fire in self-defence. Brown was shot at least six times.

Brown is suspected of having stolen cigars from a nearby convenience store shortly before the incident. Brown and a friend had been walking down the middle of the street when Wilson approached them, though police said in August that Wilson was not aware of the robbery at the time.