The US Justice Department have said they will seek the execution of a white man accused of killing nine black churchgoers in South Carolina.

The decision by federal prosecutors means Dylann Roof, who allegedly opened fire inside a church in a racially motivated attack last June, will face two death penalty trials.

In a short statement, the attorney general Loretta Lynch said: "The nature of the alleged crime and the resulting harm compelled this decision".

As well as facing nine counts of murder in state court, the 22-year-old has been charged with 33 federal crimes - including obstruction of religion, firearms offences, and hate crimes.

Lynch said her department had thoroughly considered "all relevant factual and legal issues" in its decision to seek capital punishment if he is convicted.

Federal executions are exceedingly rare in the US. Only three have been performed in the past 50 years, and none have taken place since 2003.

According to his lawyers, Roof would be prepared to plead guilty if he did not face the death penalty.

The shootings during a Bible study session at the Emanuel African Methodist Episcopal Church in Charleston sparked a nationwide debate about race in America.

At the time, Ms Lynch said: "To carry out these twin goals of fanning racial flames and exacting revenge, Roof further decided to seek out and murder African-Americans because of their race".

Roof's state trial is expected to begin in January, and a date for the federal case has not been set.

Not all of the victims' families believe Roof should face the death penalty if he is convicted, mainly because of their religious views.

But Steve Schmutz, a lawyer representing three families, said his clients "support whatever decision the US government is making in this case, and I'm sure they support this decision".