A man has been shot and killed by police in north St Louis, Missouri.

Authorities confirmed that police were involved in a shooting that left one man dead.

Police said the man was a 23-year-old accused of stealing muffins and energy drinks from a local store. 

St Louis Police Chief Sam Dotson said in a tweet that the suspect was brandishing a knife at the time of the shooting.

Witnesses said he was acting erratically.

Two officers at the scene reportedly asked the suspect to put down the weapon.

Police said he yelled, "Shoot me now, shoot me now" before charging the officers with the knife, and was then fatally shot by two police officers.

A local previously told CBS News that police said the victim was black.

An employee at a nearby market told the station that the incident took place outside the Six Stars Market along Riverview Blvd.

No officers were injured in the shooting.

The fatal shooting occurred approximately four miles from the deadly officer-involved shooting of 18-year-old Michael Brown ten days ago in Ferguson.

Two people shot in Ferguson

Two people were shot and wounded and 31 arrested overnight in the latest unrest in the Missouri town of Ferguson rocked by a racially charged killing, police said.

The wounded were hit by gunfire from demonstrators, as police did not open fire, Captain Ron Johnson of the Missouri Highway Patrol said.

Police fired tear gas in the town where a white police officer shot and killed an unarmed black teenager, just hours after US President Barack Obama called for calm.

US National Guard troops arrived in Ferguson earlier in the day.

They kept a low profile as police in riot gear dispersed about 100 demonstrators around 11pm (5am Irish time), arresting several of them.

Several rounds of gunfire, described by a senior police officer as "potshots in the area", were heard, adding to tension that has gripped this St Louis suburb since the fatal shooting of 18-year-old student Michael Brown.

His death, and the heavy-handed police response, has reignited a national debate about race and law enforcement in the United States.

Mr Obama said he was sending Attorney General Eric Holder to Ferguson tomorrow as he pursues a civil rights investigation into the case.

Mr Obama said there was no excuse for local police to employ "excessive force" and urged the state of Missouri to make only "limited" use of the National Guard.

By late afternoon, about 200 Missouri National Guard soldiers arrived in Ferguson, with snipers seen posted on rooftops near a police command centre.

The troops are operating under Missouri Highway Patrol supervision.

The reinforcements allowed State Governor Jay Nixon to lift an overnight curfew, but tempers were still running high amid controversy over Mr Brown's death.

"They're supposed to protect the American citizens, but they're fighting a war with unarmed citizens," said Ron Henry, who wore a T-shirt with the phrase "stop killing us."

Amid the trouble, Getty Images photographer Scott Olson was arrested for unknown reasons, the agency said. He was later released.

As night fell, several hundred people took part in fresh protests after police with plastic handcuffs moved crowds of demonstrators into designated zones to clear a main street in the town that had been the scene of violent protests.

Protesters were not allowed stop and gather, and were directed to keep moving.

Residents were on edge, after the previous night saw rioters loot stores and throw Molotov cocktails.

Mr Brown was shot dead on a residential street last Saturday by Darren Wilson, a 28-year-old white police officer.

A forensic pathologist retained by the victim's family said that the student had been shot at least six times - twice in the head.

Different versions of the shooting have emerged, with police sources saying there was a scuffle during which Mr Brown tried to seize the policeman's weapon, while witnesses have alleged that Mr Brown had his hands up and was not resisting when he was shot.

The Washington Post said traces of marijuana were found in Mr Brown's system.

A total of three autopsies have been requested, by local authorities, the family and the Justice Department.

Officials told news media that a Missouri grand jury could hear evidence in the case as early as tomorrow. 

Mr Obama warned of a "gulf of mistrust" between residents and police in many cities and towns across the US, particularly in those where racial minorities feel excluded from opportunities for a better life.

"To a community in Ferguson that is rightly hurting and looking for answers, let me call once again for us to seek some understanding rather than simply holler at each other," he said.

"In too many communities, too many young men of colour are left behind and seen only as objects of fear," Mr Obama said.

Former New York chief medical examiner Michael Baden, who examined Mr Brown's body on behalf of his family, said yesterday he found no evidence of an alleged struggle between Brown and the officer.

Mr Wilson is said to have been hurt in the incident, and Mr Baden said that he had not examined the police officer.

The absence of gunpowder on Mr Brown's body indicated that the muzzle of the gun was probably at least 30-60cm away, or as much as 10m, Mr Baden added.

He stressed that his findings were preliminary.