The governor of the US state of Missouri has activated the state’s National Guard in anticipation of a grand jury decision over the killing of an unarmed black teenager.

In a statement, Jay Nixon said the guard will “support law enforcement’s efforts to maintain peace”.

A panel is deciding whether to charge Officer Darren Wilson in the August death of Michael Brown, 18.

No specific date for the jury’s decision has been given.

The St Louis County prosecutor has said he expects the grand jury to reach a decision in mid-to-late November.