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. Michael Brown, 18, was shot and killed by Mr Wilson in August after a confrontation in Ferguson, a majority-black suburb of St Louis.

Witnesses say Brown had his hands up in apparent surrender to the officer when he was shot.

Police have said there was a struggle between the teenager and the officer before the shooting.His death set off protests, sometimes violent, in the area. Ferguson police and other law enforcement were criticised for being heavily armed and using excessive force during protests.