A man facing the death penalty for committing two murders was executed by firing squad on Friday, the second such execution in the US state of South Carolina this year.

Mikal Mahdi, 42, was executed for the 2004 murder of 56-year-old James Myers, an off-duty police officer, and the murder of a convenience store employee three days earlier.

According to a statement from the prison, "the execution was performed by a three-person firing squad at 6:01 PM (2201 GMT)," with Mahdi pronounced dead four minutes later.

"Tonight, the state of South Carolina executed him by firing squad — a horrifying act that belongs in the darkest chapters of history, not in a civilized society," defense lawyer David Weiss said in a statement. "Mikal died in full view of a system that failed him at every turn — from childhood to his final breath."

Myers found Mahdi hiding in a garden shed at his home before Mahdi killed him and set the body on fire. Mahdi also pleaded guilty to murdering a convenience store clerk three days before he killed Myers.

South Carolina gives its death row inmates a choice between lethal injection, the electric chair and the firing squad. Mahdi chose the firing squad.

The first execution by firing squad in the United States in 15 years was carried out in South Carolina on 7 March, when a man convicted of murdering his ex-girlfriend's parents was put to death.

A three-person squad of Department of Corrections volunteers opens fire on the condemned man, who is restrained in a chair with a hood over his head 15 feet (five meters) away.

Mahdi had requested clemency from Governor Henry McMaster but South Carolina's Republican chief executive did not grant it, or any previous clemency petitions.

Mahdi's lawyers had argued that he had suffered his entire life. He was four when his mother fled her abusive husband, leaving the boy to be raised by his volatile mentally ill father, they said.

"Between the ages of 14 and 21, Mikal spent over 80 percent of his life in prison and lived through 8,000 hours in solitary confinement," his lawyers said.

They described Mahdi as "deeply remorseful and a dramatically different person from the confused, angry and abused youth who committed the capital crimes."

Mahdi's execution was the 12th in the  United States this year. There were 25 last year.

The vast majority of US executions since the Supreme Court reinstated the death penalty in 1976 have been performed using lethal injection.

Alabama has carried out four executions using nitrogen gas, a method that has been denounced by United Nations experts as cruel and inhumane.

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

President Donald Trump is a proponent of capital punishment and on his first day in office called for an expansion of its use "for the vilest crimes."

Attorney General Pam Bondi announced last week that federal prosecutors would seek the death penalty for Luigi Mangione, charged with the high-profile 4 December murder in New York of UnitedHealthcare CEO Brian Thompson.