A South Carolina man convicted of murdering his ex-girlfriend's parents is to be put to death by firing squad on Friday in the first such execution in the United States in 15 years.

Brad Sigmon, 67, is to be executed at a prison in Columbia, the South Carolina capital, for the 2001 murders of David and Gladys Larke, who were beaten to death with a baseball bat.

Sigmon, who confessed to the murders and admitted his guilt at trial, had a choice between lethal injection, firing squad or the electric chair as his manner of execution.

Gerald "Bo" King, one of his lawyers, said Sigmon had chosen the firing squad after being placed in an "impossible" position, forced to make an "abjectly cruel" decision about how he would die.

"Unless he elected lethal injection or the firing squad, he would die in South Carolina's ancient electric chair, which would burn and cook him alive," King said.

"But the alternative is just as monstrous," he said. "If he chose lethal injection, he risked the prolonged death suffered by all three of the men South Carolina has executed since September."

The last US firing squad execution in the United States was in Utah in 2010. Two others have also been carried out by firing squad in the western state -- in 1996 and in 1977.

The 1977 execution of convicted murderer Gary Gilmore was the basis for the 1979 book "The Executioner's Song" by Norman Mailer.

The vast majority of executions in the United States have been done by lethal injection since the Supreme Court reinstated the death penalty in 1976.

Alabama has carried out four executions recently using nitrogen gas, which has been denounced by UN experts as cruel and inhumane. The execution is performed by pumping nitrogen gas into a facemask, causing the prisoner to suffocate.

Three other US states -- Idaho, Mississippi and Oklahoma -- have joined South Carolina and Utah in authorizing the use of firing squads.

Death chamber renovated

According to the South Carolina Department of Corrections (SCDC), the death chamber at the prison where Sigmon is to be executed has been renovated to accommodate a firing squad.

Bullet-resistant glass has been placed between the witness room and execution chamber.

Sigmon will be restrained in a metal chair with a hood over his head 15 feet (five meters) away from a wall with a rectangular opening.

A three-person firing squad of SCDC volunteers will shoot through the opening.

All three rifles will have live ammunition.

An "aim point" will be placed above Sigmon's heart by a member of the execution team.

There have been five executions in the United States this year and there were 25 last year.

The death penalty has been abolished in 23 of the 50 US states, while three others -- California, Oregon and Pennsylvania -- have moratoriums in place.

Three states -- Arizona, Ohio and Tennessee -- that had paused executions have recently announced plans to resume them.

President Donald Trump is a proponent of capital punishment and on his first day in the White House he called for an expansion of its use "for the vilest crimes." (AFP)

? Convicted murderer Brad Sigmon, 67, is to be put to death by firing squad in South Carolina in the first such execution in the United States since 2010 ? HANDOUT / South Carolina Department of Corrections/AFP

Update news