The US Justice Department is seeking the death penalty against a man charged with killing nine black parishioners last year in a South Carolina church.

Attorney General Loretta Lynch said the nature of Dylann Roof's alleged crimes led to the department's decision, and added that officials had considered "all relevant factual and legal issues".

Roof is awaiting trial on federal hate crime charges in connection with the shooting at Emanuel AME Church in Charleston on June 17 last year. He is also charged with nine counts of murder in state court.

Roof, who is white, appeared in photos waving Confederate flags and burning or desecrating US flags.

He allegedly wrote about stirring up racial violence, and survivors told police that he hurled racial insults during the attack. Roof was arrested a day after the shootings after a motorist spotted his Confederate number plate.

Federal prosecutors charged Roof with hate crimes one month after the shooting, saying that he was motivated by racial hatred and a desire to commit a "notorious attack".

South Carolina prosecutors have already announced plans to seek a death when Roof stands trial next year.

Federal executions are rare, and President Barack Obama has said he is "deeply concerned" about the implementation of the death penalty.

Roof's solicitors in the federal case have said the defendant would be willing to plead guilty if the death penalty is not put forward as an option.

Roof's friend Joey Meek pleaded guilty last month to lying to federal authorities, and has agreed to help with the prosecution against the defendant.