US President Barack Obama has appealed for calm after what he called the "heartbreaking" death of a black teenager killed in a police shooting in Missouri.

It comes after police fired tear gas after rioting broke out for a second night in Ferguson, Missouri.

The violence erupted despite calls for calm from the teenager's mother.

Michael Brown,18, was shot dead in Ferguson, a mostly black St Louis suburb, on Saturday afternoon after what police said was a struggle with a gun in a police car.

A witness in the case told local media Mr Brown had raised his arms to police to show he was unarmed before being killed.

The FBI has opened an investigation into the racially charged case.

"The death of Michael Brown is heartbreaking, and Michelle and I send our deepest condolences to his family and his community at this very difficult time," Mr Obama said in a statement.

"I know the events of the past few days have prompted strong passions, but as details unfold, I urge everyone in Ferguson, Missouri, and across the country, to remember this young man through reflection and understanding," Mr Obama added.

Mr Brown's family has hired attorney Benjamin Crump, who represented the family of Trayvon Martin, a black teenager who was shot dead by a community watch volunteer in 2012.

Police in riot gear fired tear gas to disperse a crowd estimated in the hundreds gathered near a building that burned during rioting on Sunday night.

Fire trucks, ambulances and more officers converged on the area in a chaotic scene.