The US Department of Justice is preparing to sue the Ferguson, Missouri, police department over allegations of racially discriminatory practices unless the police force agrees to make changes, CNN reported on Wednesday.

The network said the Justice Department would not charge the white Ferguson police officer involved in the fatal shooting of unarmed black teenager Michael Brown last August but was expected to outline allegations of discriminatory Ferguson police tactics.

The department would file suit if Ferguson police did not agree to review and change those tactics, CNN reported.

The shooting of Brown last August by officer Darren Wilson led to months of sometimes violent protests in Ferguson and galvanised critics of the treatment by police and the US criminal justice system of blacks and other minority groups.

A St. Louis County grand jury decided last year not to prosecute Wilson, who has since left the Ferguson police force. The Justice Department has been conducting probes of the shooting and the operations of the Ferguson police force.

Justice Department spokesman Peter Carr declined to comment on the CNN report.

US Attorney General Eric Holder, who is preparing to leave office, said earlier this month he hoped to complete the civil rights investigation of the shooting before he steps down.

CNN said the potential Justice Department lawsuit could include allegations that police targeted minorities in issuing minor traffic infractions and then jailed them if they could not pay the fines.

It reported the agency would seek court supervision of changes taken by Ferguson police to improve its dealings with minorities.