The state also promised more to come in Alabama: Attorney General Steve Marshall said that 43 other people on death row had chosen asphyxiation over lethal injections since lawmakers approved the method in 2018.
Alabama called the new method "humane," while human rights groups condemned it as cruel and torturous.
Kenneth Smith was given the death penalty for a 1988 murder-for-hire slaying of a preacher's wife. (AP PHOTO)
"Alabama has done it, and now so can you, and we stand ready to assist you in implementing this method in your states," Marshall, a Republican, told reporters on Friday. Oklahoma and Mississippi politicians have also added nitrogen asphyxiation to their states' execution methods, but have not yet used it.
Marshall said asphyxiation by nitrogen, the first new execution method since lethal injections began in the US in 1982, is "no longer an untested method."
"It is a proven one," he said.
There were diverging accounts as to how violent the method was between state officials and some who witnessed the public execution of Smith, who, unusually, survived a first execution attempt in 2022 when executioners struggled to insert an intravenous line for a lethal injection.
Alabama had predicted in court filings that, under its new method, Smith would slip into unconsciousness within about 30 seconds and die soon after. Executioners strapped a commercial industrial-safety respirator mask over the man's face and connected it to a canister of pure nitrogen.
Five journalists allowed to watch the execution through a window as media witnesses said he remained conscious for several minutes after the nitrogen flowed, and then began shaking and writhing on the gurney for about two minutes.
The Reverand Jeff Hood, who stood besides Smith as his spiritual adviser after signing a safety waiver acknowledging the risks of nitrogen asphyxiation, said Smith repeatedly threw his head forward as he struggled for life.
Alabama officials said everything went as expected. They said Smith appeared to hold his breath for as long as he could, and suggested the writhing could have been "involuntary movements."
"What occurred last night was textbook," Marshall said.
Kenneth Smith is the first death row inmate to be executed by nitrogen hypoxia in the US. (AP PHOTO)
Alabama's Department of Forensic Sciences will perform an autopsy on Smith's body, prison officials said.
Smith was convicted of murdering Elizabeth Sennett after accepting $US1,000 ($A1,517) to kill her with accomplices at the behest of her husband, a preacher who later killed himself.
The jury voted 11-1 to sentence him to life in prison but an Alabama judge overruled their decision under a law that was later scrapped and ruled unconstitutional. Some of Sennett's relatives witnessed the execution, and told reporters afterwards that they had forgiven her killers and were glad the execution was over.
The American Civil Liberties Union, Amnesty International USA and other rights groups condemned the execution.
"The whole purpose of these methods is to hide pain," said Maya Foa, joint executive director of the rights advocacy group Reprieve.
"How many more prisoners must die agonising deaths before we see executions for what they really are: the state violently taking a human life?"
Smith unsuccessfully challenged Alabama's attempts to kill him in federal courts, arguing the botched first effort and the untested new method violated a constitutional ban on "cruel and unusual punishment."
