The man accused of shooting two police officers in Ferguson was not targeting officers or aiming at demonstrators, his lawyer said as he countered a police description of the crime that has heightened tensions in the city.

A man, thought to be a black teenager, has been shot dead by a police officer in the same city where the killing of Michael Brown sparked protests.

The white officer who shot dead an unarmed black teenager does not expect to face criminal charges from a Missouri grand jury investigating the case, a police union has said.

Missouri state police are taking over security from local officers in the St Louis suburb that has been the scene of violent protests since an unarmed black teenager was shot dead by white police officer.

Violence again erupted in the St Louis area near the site of the police shooting death of an unarmed black teenager, according to local police, despite calls by US President Barack Obama and activists for a measured response.