Atmore, Alabama - A convicted murderer is scheduled to be executed in Alabama this week using nitrogen gas, a novel method that the United Nations has likened to "torture."

Kenneth Eugene Smith (58) has been on death row for more than three decades after being convicted in 1989 of a murder-for-hire.

Smith is to be put to death at Holman Prison in Atmore, Alabama, during a 30-hour window beginning at 1:00 AM EST on Thursday.

He is to be executed by nitrogen hypoxia, a method that has not been used before in the United States.

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There were 24 executions in the United States in 2023, all of them carried out using lethal injection.

Smith was subjected to a failed execution attempt in 2022 when prison officials were unable to set intravenous lines to administer a lethal injection.

The last US execution using gas was carried out in 1999 when a convicted murderer was put to death using hydrogen cyanide gas.

Alabama is one of three US states that have approved the use of nitrogen hypoxia as a method of execution. It involves administering nitrogen gas through a facemask, depriving the body of oxygen.

Ravina Shamdasani, a spokeswoman for the UN rights office in Geneva, urged Alabama last week to abandon plans to execute Smith using the "novel and untested" method.

Shamdasani said it could "amount to torture or other cruel, inhuman or degrading treatment or punishment, under international human rights law."

Smith was convicted of the 1988 murder of Elizabeth Sennett, a pastor's wife. Her husband, Charles Sennett, who had arranged the murder, committed suicide a week after her death.

Smith appealed to the Supreme Court for a stay of execution, but the nation's highest court denied the request on Wednesday without comment.

The state of Alabama, in a filing urging the court to reject the stay, defended the method of execution, claiming that it was "perhaps the most humane method of execution ever devised."

Capital punishment has been abolished in 23 US states, while the governors of six others – Arizona, California, Ohio, Oregon, Pennsylvania and Tennessee – have put a hold on its use.