Alabama has carried out the first execution of a death row inmate in the US using nitrogen gas, an untested procedure which the prisoner’s lawyers had argued amounted to a form of cruel and unusual punishment banned under the US constitution.

Kenneth Smith, 58, was pronounced dead at 8.25pm on Thursday evening at an Alabama prison after breathing pure nitrogen gas through a face mask to cause oxygen deprivation.

The execution had been scheduled to begin at 6pm local time at the Holman Correctional Facility in Atmore, Alabama, but it was delayed as the US supreme court weighed his final appeal. Shortly before 8pm, the court denied that appeal, allowing the execution to proceed.

Justice Sonia Sotomayor, who along with two other liberal justices dissented, wrote: “Having failed to kill Smith on its first attempt, Alabama has selected him as its ‘guinea pig’ to test a method of execution never attempted before. The world is watching.”

Smith’s lawyers had argued that to go ahead with executing him under these untried conditions would violate constitutional protections against cruelty.

The prisoner also unsuccessfully argued that he was being dealt with doubly unlawfully by dint of him having been subjected to an execution procedure once before. In November 2022, the state strapped him for four hours to the gurney and punctured his arms and legs in a failed attempt to find a vein through which to kill him using lethal medications.

That placed Smith in a highly rare class of an inmate who could describe what it was like to survive an execution.

The new death protocol adopted by Alabama and used to kill Smith involved placing an industrial-style respirator mask over the prisoner’s head and then forcing him to breathe pure nitrogen. The technique, known as “nitrogen hypoxia”, leads to fatal oxygen deprivation.

Days before he was executed, Smith told the Guardian in a phone call from his prison cell that he was not ready to die. He had been diagnosed with PTSD caused by his first failed execution attempt, and was suffering from sleeplessness and anxiety.

He said he was terrified by the prospect of vomiting in the mask leading to death by drowning on the contents of his own stomach, a gruesome possibility that was raised by his lawyers in court.

In his Guardian interview, Smith also appealed to the American people to show mercy for those like him facing judicial killings. “You know, brother, I’d say, ‘Leave room for mercy’. That just doesn’t exist in Alabama. Mercy really doesn’t exist in this country when it comes to difficult situations like mine,” he said.

Smith was convicted of the 1988 murder-for-hire of a pastor’s wife, Elizabeth Sennett. He and another man were each allegedly paid $1,000 to kill her by Charles Sennett, a minister in the Church of Christ who went on to take his own life after suspicion fell on him.

In the run-up to the execution, Alabama had come under a raft of domestic and international criticism. Hundreds of Jewish clergy and community leaders across the US signed a letter organised by L’chaim!, Jews Against the Death Penalty, calling for a halt.

“Just the idea of using gas for executions is an affront to our community,” the co-founder of L’chaim, Mike Zoosman, said. “The Nazi legacy of experimentation to find the most expeditious way to rid our community of undesirable prisoners is an undercurrent for anyone who is aware of that history that should not be repeated in Alabama, or anywhere.”

Smith’s pending death was also denounced by UN experts on arbitrary executions and torture who fiercely opposed the use of what they decried as a human experiment.

In his Guardian interview before his death Smith said he feared that if Alabama carried out his execution it would put the new killing method of nitrogen gas on the map. He had a warning for his fellow Americans.

“I fear that it will be successful, and you will have a nitrogen system coming to your state very soon,” he said.

The Associated Press contributed reporting