A grand jury has reached a decision about whether to charge a white police officer in the shooting dead of Michael Brown, an unarmed black 18-year-old, US prosecutors say.

The US National Guard has begun withdrawing from Ferguson, where there has been unrest since a white police officer fatally shot an unarmed black 18-year-old nearly two weeks ago.

US police officers in St Louis shot and killed a 23-year-old man who they say was wielding a knife and refused to drop it.