A grand jury has reached a decision on whether to indict a white police officer who fatally shot Michael Brown, an unarmed black 18-year-old, a spokesman for St Louis County's top prosecutor has said.

The panel has been considering charges against Darren Wilson, the suburban St Louis officer whose fatal shooting of Brown in August sparked days of protests.

In a brief email to reporters, spokesman Ed Magee said the decision would be announced later on Monday. He offered no immediate details.

The St Louis County prosecutor's office has told the family the decision will be announced after 6 pm eastern (23GMT), attorney Benjamin Crump told The Associated Press.

At a press conference on Monday Missouri governor, Jay Nixon, called for "peace, respect and restraint" as protesters across the country planned for demonstrations even before the jury verdict was announced.

The August 9 killing of Brown reignited a debate over how police treat young black men in the US. It drew attention to racial tensions simmering in Ferguson and other US communities four decades after the civil rights movement of the 1960s.

The tensions have evoked other racially charged cases, including the riots that rocked Los Angeles in 1992 after the acquittal of white police officers in the videotaped beating of black motorist Rodney King.

More recently, peaceful protests followed the 2013 not-guilty verdict in the killing of unarmed Florida black teenager Trayvon Martin by George Zimmerman, who was not a police officer but coordinated the local neighbourhood watch.

Source: Agencies