WASHINGTON, United States (AFP)- A serial rapist and murderer is to be put to death by nitrogen gas in Alabama on Thursday, the fourth use of the controversial execution method in the southern US state.

Demetrius Frazier, 52, was sentenced to death in 1996 for the 1991 murder of 40-year-old Pauline Brown, a mother of two, in the Alabama city of Birmingham.

Frazier was found guilty of breaking into Brown’s apartment, raping her and shooting her in the head. The jury voted 10-2 to recommend the death penalty.

According to court documents, Frazier, who is Black, accused the predominantly white jury during his trial of being racist.

Frazier was previously convicted of rape and the 1992 murder of a 14-year-old girl, Crystal Kendrick, in the northern state of Michigan.

Michigan does not have the death penalty and Frazier was serving a life sentence there for Kendrick’s murder and for two separate rape convictions.

He was transferred to Alabama in 2011 and appeals seeking to have him sent back to Michigan to serve his life sentence have been rejected.

His appeals claiming that the nitrogen gas method of execution amounts to cruel and unusual punishment have also been denied.

Frazier is to be put to death at a prison in Atmore, Alabama, with the execution tentatively scheduled to begin at 6:00 pm Central Time (0000 GMT) on Thursday.

Alabama carried out three executions by nitrogen asphyxiation last year and is the only US state currently using the method. Other US states use lethal injection.

The execution is performed by pumping nitrogen gas into a facemask, causing the prisoner to suffocate.

United Nations experts appointed by the UN Human Rights Council have denounced the use of nitrogen gas, saying it “may amount to cruel, inhuman or degrading treatment, or even torture.”

There were 25 executions in the United States 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.”