A convicted murderer in the southern state of Alabama is expected on Thursday to become the first person executed in the United States using nitrogen gas, a method the UN has likened to "torture."

Kenneth Eugene Smith, 58, has been on death row for more than three decades after being convicted of the 1988 murder-for-hire of a pastor's wife.

Smith is scheduled to be put to death at Holman Prison in Atmore, Alabama, during a 30-hour window which began at 1:00 am Eastern Time (0600 GMT) on Thursday.

Smith is set to be executed by nitrogen hypoxia, which involves pumping nitrogen gas into a facemask, causing him to die from lack of oxygen.

The method has never been attempted before in the United States, according to the Death Penalty Information Center.

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

In an interview with National Public Radio (NPR) in December, Smith said he was "absolutely terrified" about his upcoming execution and still suffering "trauma" from the previous failed bid.

"Everybody is telling me that I'm going to suffer," he said.

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

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

Alabama is one of three US states that have approved the use of nitrogen hypoxia as a method of execution, along with Oklahoma and Mississippi.

Ravina Shamdasani, a spokeswoman for the UN rights office in Geneva, urged Alabama last week to abandon the plan to execute Smith using what she called a "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."

No sedation

While nitrogen gas has never been used to execute humans in the United States, it is sometimes used to kill animals.

But Shamdasani pointed out that the American Veterinary Medical Association recommends giving even large animals a sedative when being euthanized in this manner.

Alabama's protocol for execution by nitrogen asphyxiation makes no provision for sedation prior to execution.

The state of Alabama has defended the method of execution, claiming that it is "perhaps the most humane method of execution ever devised."

Beyond the execution method, Shamdasani reiterated the UN's opposition to the death penalty in principle.

"The death penalty is inconsistent with the fundamental right to life," she said.

Smith and an accomplice, John Parker, were convicted of the 1988 murder of Elizabeth Sennett for which they were each paid $1,000.

Sennett's husband, Charles Sennett, who had arranged his wife's murder, killed himself a week after her death. Parker was executed by lethal injection in 2010.

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

Further appeals for a stay are pending but the conservative-dominated court has rarely granted such requests in recent years.

According to a recent Gallup Poll, 53 percent of Americans support the death penalty for someone convicted of murder, the lowest level since 1972.

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.