Missouri's governor has declared a state of emergency and activated the National Guard ahead of a grand jury's decision on whether to charge a white police officer for the shooting death of an unarmed black teenager.

Jay Nixon on Monday signed an executive order activating the state's National Guard to assist state and local police in Ferguson as needed.

The St Louis suburb has been on edge for months as a grand jury heard evidence in the 9 August shooting death of Michael Brown.

Missouri officials have urged residents not to riot if a decision is made not to charge Ferguson Police Officer Darren Wilson.

Gov Nixon said last week his priority was to keep residents and businesses safe.

"As part of our ongoing efforts to plan and be prepared for any contingency, it is necessary to have these resources in place in advance of any announcement of the grand jury’s decision," he said in a statement announcing the activation of Guard troops.

"These additional resources will support law enforcement's efforts to maintain peace and protect those exercising their right to free speech."

There was no indication that a decision from the grand jury was imminent.

The St Louis County prosecutor said he expects jurors to reach a decision in mid-to-late November.

Several business owners have boarded up their shops amid fears there could be a repeat of the sometimes violent protests that erupted in the wake of Mr Brown's death.

Varying accounts of the shooting have emerged from witnesses; some described a struggle between the pair, while others said the teenager had put his hands up.

Mr Brown was shot at least six times, twice in the head, on the street where he lived, according to pathologist Dr Michael Baden, who was hired by the Brown family.