Image copyright Reuters Image caption Protests, sometimes violent, erupted after the police killing of Michael Brown in Ferguson

The US Justice Department is suing Ferguson, Missouri to force the city to adopt police reforms negotiated with the federal government.

Unarmed black teenager Michael Brown was shot dead by a police officer in the city in 2014, sparking protests.

Ferguson was required to reform its policing after investigators found widespread racial bias in the force.

US Attorney General Loretta Lynch said there was no option but to sue after the city voted to revise the agreement.

"The residents of Ferguson have waited nearly a year for their city to adopt an agreement that would protect their rights and keep them safe," she said.

"They should not be forced to wait any longer."

She accused Ferguson's police of routinely carrying out unconstitutional practices that disproportionately targeted the black community.

Image copyright Family handout Image caption Michael Brown's death put the spotlight on issues of race and policing in the US

Ferguson's council voted six to none on Tuesday night to reject the agreement, citing concerns about costs.

Even if they win the civil rights suit filed against them, they could face millions of dollars in legal fees.

There has been no immediate comment from Ferguson officials on the justice department's announcement.

A grand jury and the Department of Justice declined to prosecute Michael Brown's killer, white officer Darren Wilson.

The shooting generated a national debate about race relations and police violence.