PORTLAND, Ore. (Reuters) - The Portland police must reinstate a white officer who was fired in 2010 after he fatally shot an unarmed black man in the back, an Oregon state appeals court has ruled, officials said on Thursday.

Portland Police Bureau officer Ron Frashour was fired for shooting 25-year-old Aaron Campbell after the suspect's girlfriend had called police to her apartment over concerns that he was suicidal.

The Oregon Court of Appeals sided with a labor dispute arbitrator in ruling on Wednesday that the city must rehire Frashour.

It was unclear when Frashour would return to active duty, and in what role, though the police department said he would undergo extensive training.

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The ruling comes at a time when most U.S. police receive little or no training on how best to handle crises involving the mentally ill despite the growing frequency of such encounters and the fatal results in a number of recent cases.

It also came the same day Chicago Mayor Rahm Emanuel told reporters his police force would get new training to defuse tense situations and limit use of lethal force amid protests over fatal police shootings.

Shootings of black men by police officers across the country over the past 18 months have led to national protests and the issue has fueled a civil rights movement under the name Black Lives Matter.

In Oregon, after Frashour was terminated, he filed a grievance through the city's police union and in 2012 a state arbitrator ruled that he should be reinstated, according to court records.

He was then paid back wages and placed on paid administrative leave while Portland appealed that ruling.

The court said Portland's argument that the arbitrator made a "mistake of law or fact" in the way she interpreted the department's use-of-force policies was not legal grounds for overturning the arbitrator's decision.

"I am disappointed the court blocked the city's efforts to terminate an officer who used force inappropriately," Portland Mayor Charlie Hales said.

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The Albina Ministerial Alliance, a community group that for years has advocated for police reform in Portland, decried the appeals court ruling and asked that Frashour be assigned to a position that minimizes his contact with the public.

Neither Frashour, nor an attorney representing him, could immediately be reached for comment.

(Reporting by Courtney Sherwood in Portland, Oregon; Editing by Eric M. Johnson and David Gregorio)