South Carolina will seek the death penalty for Dylann Roof, who is charged with the June murders of nine black worshippers at a Charleston church, a state prosecutor said on Thursday.

The 21-year-old Roof, who is white, has been accused of gunning down his victims, members of a Bible study group at Charleston's Emanuel AME Church, because it was a nationally known historically black church.

"This was the ultimate crime and justice from our state calls for the ultimate punishment," Ninth Judicial Circuit Solicitor Scarlett Wilson said at a news conference.

She acknowledged that many of the victims' relatives had spoken of a willingness to forgive Roof during his first court appearance.

"Forgiveness does not necessarily mean foregoing consequences, even severe consequences," she said.

Andy Savage, a Charleston attorney for some of the victims families as well as three survivors, said that while his clients are not advocates of the death penalty, "at the same time they recognize that the needs of the state are different".

She said some of the victims' families did not believe in the death penalty for religious reasons, and others felt it was too easy, there were still others that felt it was appropriate punishment. Wilson said all had shown respect for her decision.

The death penalty decision could determine how Roof pleads. He has not entered a plea so far in the state's murder case.

At his July 31 arraignment on separate federal hate crime and firearms charges, his attorney said Roof wanted to plead guilty, but the defence team was not ready to let him do so.

"Until we know whether the government will seek the death penalty, we cannot advise Mr Roof," defense attorney David Bruck told the court at the time. The federal judge then entered a temporary "not guilty" plea on the court's behalf.

Some of the victims' relatives have said they forgive Roof but have not said where they stand on the death penalty.

Roof is accused of shooting the "Emanuel 9" on June 17 after he had entered the church, joined their group and sat quietly with them before opening fire.

Among the dead was Clementa Pinckney, the church's pastor who was also a state senator. President Barack Obama eulogized Pinckney in Charleston at a funeral attended by more than 5,000 people.