“I deeply regret the execution of Kenneth Eugene Smith in Alabama despite serious concerns that this novel and untested method of suffocation may amount to torture, cruel, inhuman or degrading treatment,” said Volker Türk, the UN High Commissioner for Human Rights, OHCHR.

Convicted of murder in 1988, Mr. Smith, 58, had been sentenced to death in the state of Alabama. An attempt to carry out the execution in 2022, failed.

Last week, the Human Rights Council had called for Alabama to halt Mr. Smith’s execution and to refrain from taking steps towards any other executions using nitrogen asphyxiation.

On Thursday evening, nitrogen hypoxia gas was administered to Mr. Smith.

It took 22 minutes before he was formally declared dead, media reports said.

According to media and other witnesses in the execution chamber, the prisoner shook violently, writhing and convulsing on the gurney.

Prior to the execution, authorities in Alabama reportedly described the use of pure nitrogen gas as “perhaps the most humane method of execution ever devised”.

“The death penalty is inconsistent with the fundamental right to life,” said OHCHR Spokesperson Ravina Shamdasani on Friday morning in a post on X, formerly Twitter.

“We urge all States to put in place a moratorium on its use as a step towards universal abolition.”