The US state of Missouri has declared a state of emergency and activated the National Guard ahead of a grand jury decision about whether a white police officer will be charged in the fatal shooting of a black teenager in the St Louis suburb of Ferguson.

Missouri governour Jay Nixon said on Monday the National Guard would assist state and local police in case the grand jury's decision leads to a resurgence of the civil unrest that occurred in the days immediately after the August 9 shooting of Michael Brown by Ferguson Police Officer Darren Wilson.

"All people in the St Louis region deserve to feel safe in their communities and to make their voices heard without fear of violence or intimidation," Nixon said in a written statement.

There is no specific date for a grand jury decision to be revealed, and Nixon gave no indication that an announcement is imminent. But prosecutor Bob McCulloch has said that he expects the grand jury to reach a decision in mid-to-late November.

The grand jury is considering whether there is enough evidence to proceed with charges and, if so, what those charges should be.

If the jury issues an indictment, a separate jury will be selected to decide whether the person is guilty.

Separate investigation

The US Justice Department, which is conducting a separate investigation, has not said when its work will be completed.

Brown, who was unarmed, was shot by Wilson after some sort of confrontation occurred when the officer spotted Brown and a friend and told them to stop walking in the middle of a street.

Brown's shooting stirred long-simmering racial tensions in the St Louis suburb, where two-thirds of the residents are black but the police force is almost entirely white.

Rioting and looting a day after the shooting led police to respond to subsequent protests with a heavily armoured presence that was widely criticised for continuing to escalate tensions.

The governor also declared a state of emergency in August.