Barack Obama distanced himself from a decision to deploy Missouri national guard troops to deal with continued unrest in Ferguson on Monday, dispatching attorney general Eric Holder to monitor events and saying he would be watching whether troops are “helping or hindering the situation”.

On Monday night unrest flared again, with police returning to the use of teargas and stun grenades against protesters, according to reports. Media were being ordered to stay behind a police-designated cordon as the latest confrontations unfolded:

Chris McGreal (@ChrisMcGreal) Police in #ferguson removing reporters so no independent witnesses to what happens next against demonstrators

Missouri governor Jay Nixon earlier announced the decision to deploy the national guard to Ferguson after the worst night of violence since a police officer shot dead an unarmed teenager, Michael Brown, on 9 August. It is the first time that the national guard has been deployed to quell civil unrest in the United States since the 1992 race riots in Los Angeles.

Speaking from the White House, Obama called for a review of the use of military equipment by local police forces, warning that a continued blurring of the lines between military and local law enforcement would be “contrary to our traditions”.

Governor Nixon, struggling to find an effective response to the protests that have rocked Ferguson for more than a week, has abandoned a controversial and ineffective curfew that failed to prevent violence.

Facebook Twitter Pinterest Police officers detain a demonstrator for protesting Michael Brown’s murder in Ferguson, as National Guard troops were deployed. Photograph: Joshua Lott /Reuters

In anticipation of the first night of national guard deployment, police in Ferguson enforced sharply different tactics through the afternoon on West Florissant Avenue, the street where most clashes have taken place.

For several days, crowds and cars were allowed by Captain Ron Johnson, the Missouri highway state patrol officer placed in charge of policing the protests, to build steadily through the afternoon. The strategy left hundreds of people poised for confrontation as a midnight curfew approached.

But on Monday, county police officers were ordering people to keep moving and barring them from congregating around the burned out QuikTrip convenience store that has been a base for many protesters. Two people were arrested, apparently for unlawful assembly, outside a McDonald’s that was badly vandalised on Sunday night.

As the evening approached, a county police spokesman announced in a statement that an “organised protest zone” would be established close to the convenience store where Michael Brown allegedly stole cigars minutes before he died.

Jon Swaine (@jonswaine) The "organised protest zone" appears to have been created in #Ferguson https://t.co/CpAq6B6Rj3

Traffic would be blocked from West Florissant by 5pm, said the spokesman, Brian Schellman, preventing access for protesters who on recent nights have repeatedly driven up and down the street with passengers hanging out of windows or sitting on the roof.

In Washington, Obama struck a more detached tone over the root causes of violence. He acknowledged the feelings of alienation felt by many young African Americans in the US: “In too many communities around this country young men of color are left behind and seen as objects of fear,” he said. But he also accused some protestors of “stirring chaos” and declined to answer whether he would be visiting the city himself.

“While I understand the passions and the anger that arise over the death of Michael Brown, giving in to that anger by looting or carrying guns, and even attacking the police only serves to raise tensions and stir chaos,” said the president in a White House address that interrupted his two-week vacation. “It undermines rather than advanc[es] justice.”

Earlier on Monday, it was revealed that the White House had not been given advance warning of governor Jay Nixon’s decision to call out the Missouri national guard, something Obama did not deny when asked about the deployment.

“I spoke to Jay Nixon about this, expressed an interest in making sure that if in fact the national guard is used, it is used in a limited and appropriate way,” said the president. “He described the support role that they’re going to be providing to local law enforcement. And I’ll be watching over the next several days, to assess whether, in fact, it’s helping rather than hindering progress in Ferguson.”

A heavy-handed police response in the first few days of demonstrations has been widely criticised in Washington, where some lawmakers are proposing legislation to limit a federal scheme that has been used to distribute surplus military equipment to civil police departments. .

“I think it’s probably useful for us to review how the funding has gone, how local law enforcement has used grant dollars, to make sure that what they’re – what they’re purchasing is stuff that they actually need,” said Obama. “Because, you know, there is a big difference between our military and our local law enforcement, and we don’t want those lines blurred. That would be contrary to our traditions.”

But, other than announcing that Holder would be visiting Ferguson on Wednesday, there was little new intervention by the administration in the handling of the crisis.

Unlike his response to the shooting of Florida teenager Trayvon Martin, Obama has been reluctant to take sides in the dispute over the police shooting of Michael Brown – perhaps wary of further polarising the national debate or appearing to condone the violent reaction.

“I have to be very careful about not prejudging these events before investigations are completed,” said Obama.

Holder acknowledged the frustrations felt by protesters about the lack of information about the investigations. “I realise there is tremendous interest in the facts of the incident that led to Michael Brown’s death, but I ask for the public’s patience as we conduct this investigation,” Holder said in a statement after meeting Obama.

Facebook Twitter Pinterest Dr Michael Baden points to an autopsy diagram showing where the gun shots hit Michael Brown. Photograph: Joe Raedle /Getty Images

In Ferguson, attorneys for Brown’s family claimed that the findings of a private autopsy indicated that he was killed while trying to surrender.

Brown was shot “at least” six times by officer Darren Wilson and was ultimately killed by a bullet that entered the top of his head and travelled “back to front”, attorney Daryl Parks said the autopsy had found, arguing this showed Brown was fatally shot when his head was well bowed.

Dr Michael Baden, who carried out the autopsy, said that “there weren’t signs of a struggle” on Brown’s body. Police have said that Brown assaulted Wilson after the officer stopped him and a friend and told them to walk on the sidewalk rather than in the road on 9 August.

However Baden, the former chief medical examiner for New York City, was more cautious than the lawyers, stating that the findings in his preliminary report “could be consistent” with suggestions that Brown had been shot while charging at Wilson. “It’s possible,” he said.

“There are many different witness testimonies,” said Baden. “Many seem to line up in one direction, some in another direction. Right now until we get more information we can’t, from a forensic science point of view, can’t distinguish and can’t make an absolute judgment.”