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’s governor has declared a state of emergency and mobilised the National Guard as a grand jury is poised to decide whether a white police officer will be charged over the fatal shooting

The police chief in the St Louis suburb where an officer fatally shot an unarmed teenager has identified the officer as Darren Wilson.

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.