US President Barack Obama has expressed sympathy for the "passions and anger" sparked by the police killing of an unarmed black teenager, but decried the violence that has rocked the St Louis suburb of Ferguson since then.

During a brief pause in his summer vacation on Monday, Obama said overcoming the mistrust endemic between many communities and their local police would require Americans to "listen and not just shout".

"That's how we're going to move forward together, by trying to unite each other and understand each other and not simply divide ourselves from one another," Obama told reporters at the White House.

"We're going to have to hold tight to those values."

Michael Brown's fatal shooting by the police on August 9 while he was walking down the street unarmed in the St Louis suburb has led to days of protests and a heavy-handed police response.

Fresh clashes erupted on Monday night, with police firing tear gas and stun grenades at rowdy crowd of protesters.

Police in riot gear, supported by an armoured SWAT vehicle and a helicopter overhead, had repeatedly ordered the crowd to disperse.

Speaking after meeting with Attorney General Eric Holder, Obama said Holder would travel to Ferguson to meet with FBI and other officials carrying out an independent federal investigation into Brown's death.

His statement followed claims the black teenager had been shot at least six times.

Shot several times

"Six bullets struck, and two may have re-entered" Brown's body, said Michael Baden, a former New York chief medical examiner, on Monday. Baden was asked by Brown's family and lawyers to conduct the independent examination on his remains.

One of the bullets hit the top of Brown's head, another struck his eye, while others hit his right arm, Baden told a news conference in Ferguson.

"All of the gunshot wounds could have been survivable, except the one at the top of the head," he said.

The family said that the bullet wound on the top of the skull suggested Brown was bowing his head to submit to the officer when he was shot.

Baden said he had found no evidence of an alleged struggle between Brown and the officer, who was reported to have been hurt in the incident, but added that he had not examined the policeman.

The absence of gunpowder on Brown's body indicated that the muzzle of the gun could have been as close as a metre or as far as nine metres away, Baden added.

He stressed his findings were preliminary and that he needed to see X-rays taken by local coroners just before the bullets were removed from Brown's corpse.

The Baden findings were released as the US National Guard began deploying in Ferguson as Missouri's governor lifted a curfew imposed over the protests.

"With these additional resources in place, the Missouri state highway patrol and local law enforcement will continue to respond appropriately to incidents of lawlessness and violence, and protect the civil rights of all peaceful citizens to make their voices heard. We will not use a curfew tonight," Governor Nixon said in a statement.

UN chief Ban Ki-moon on Monday called on US authorities to protect the right of peaceful protests over Brown's death in Ferguson.

"He calls on all to exercise restraint, for law enforcement officials to abide by US and international standards in dealing with demonstrators," a UN spokesman said.