A federal judge on Monday barred further elections for the Ferguson, Missouri, school district until it reforms a system he said violated the rights of black voters in a city that has become the face of a fierce US debate on race.

US District Judge Rodney Sippel wrote in a 119-page order that while there was no intentional discrimination at play in the Ferguson-Florissant School District elections, a number of factors including racially polarised voting patterns combined to effectively thwart black candidates.

"The political processes for electing board members in the Ferguson-Florissant School District deprives African-American voters of an equal opportunity to elect representatives of their choice," Sippel wrote in response to a lawsuit filed by the American Civil Liberties Union on behalf of the NAACP civil rights organisation.

The mostly black suburb of St. Louis became the focus of international attention in 2014 after a white police officer fatally shot Michael Brown, an unarmed black teenager. The incident sparked protests across the country against police treatment of minorities, giving rise to the Black Lives Matter movement.

Cindy Ormsby, an attorney for the school district, said the district was "very disappointed" with the ruling and was considering an appeal. She said the current board was representative of the racial composition of the community and that African-Americans had won a seat on the board in the past three years.

"The Board is united in the belief that this at-large system does not violate Section 2 of the Voting Rights Act," she said.

Jeffrey Mittman, director of the Missouri ACLU, called the ruling an important step toward remedying "the long history of governmental policies in Missouri that have worked to disfavor communities of color."

Sippel's order noted that three of the board's seven seats were held by African-Americans, while the student body for the district was predominantly black and the voting-age population in the area was about 50 percent African-American.

The judge also said that since 2004, white candidates had won election to the school board at a rate of nearly 70 percent, while black candidates won only about 11 percent of the time.

"There is a history of officially sanctioned discrimination in the region and the district, and that history is not just a distant memory," Sippel added.

He said that in addition to racially polarised voting where white voters would not back black candidates, factors like subtle racial campaign appeals combined to weaken the African-American vote.

Ferguson-Florissant uses an at-large system for electing school board members, meaning the entire region votes when a member's term is up.