President Barack Obama has appealed for "peace and calm" on the streets of a St Louis suburb besieged by violent clashes between police and crowds protesting the shooting death of an unarmed teenager.

"I know emotions are raw right now in Ferguson and there are certainly passionate differences about what has happened," Obama said in his first in-person remarks about the tense standoff in the Missouri suburb on Thursday. "But let's remember that we're all part of one American family, we are united in common values and that includes the belief in equality under the law, respect for public order and the right to peaceful public protests."

In brief remarks near his vacation spot in Martha's Vineyard, Obama said he wanted an open and transparent investigation into the shooting death of 18-year-old Michael Brown, so that justice is done.

"Now is the time for healing. Now is the time for peace and calm on the streets of Ferguson. Now is the time for an open and transparent process to see that justice is done," Obama said.

Obama also said there was no excuse for violence against the police. He added that police shouldn't be arresting and bullying journalists who are doing their jobs.

Two reporters were taken into custody and briefly jailed on Wednesday evening in the St Louis suburb.

Protesters have gathered every night in Ferguson, the mostly black suburb of St Louis, since Brown was fatally shot on Saturday during what authorities said was a struggle over a gun in a police car. Some witnesses say he was outside the car with his hands up.

The Missouri incident recalls Obama's emotional reaction to the shooting of Trayvon Martin, the black teenager who was shot to death by a white Hispanic neighbourhood watch volunteer in Florida in 2012. 

The president has faced criticism since arriving on Martha's Vineyard for spending time golfing or partying with friends
while US forces were active in Iraq and mayhem erupted in Ferguson.

The White House rejects that criticism and has said Obama continues to do his job while having some down time to relax.

Shortly after completing his statement on the situations in Ferguson and Iraq, Obama went golfing with friends.

Source: Agencies