Alabama executed a man using the controversial method of nitrogen asphyxiation on Thursday.

Convicted murderer Kenneth Eugene Smith was pronounced dead on Thursday evening after breathing pure nitrogen gas through a face mask to cause oxygen deprivation.

The execution took around 22 minutes.

Witnesses report shaking and writhing

For at least two minutes, Smith appeared to shake on the gurney, sometimes pulling against the restraints. That was followed by several minutes of heavy breathing, witnesses said.

"It appeared that Smith was holding his breath as long as he could," Alabama Corrections Commissioner John Hamm said when asked about the shaking at a press conference.

"He struggled against the restraints a little bit but it's an involuntary movement and some agonal breathing. So that was all expected."

It was the first time a new execution method has been used in the US since the lethal injection was introduced in 1982.

"Tonight Alabama causes humanity to take a step backwards," Smith said in his final message. "I'm leaving with love, peace and light."

Why was the execution so controversial?

Alabama Attorney-General Steve Marshall claimed that nitrogen asphyxiation is "perhaps the most humane method of execution ever devised."

However, Ravina Shamdasani, a spokesperson for the UN rights office in Geneva, warned the method could "amount to torture or other cruel, inhuman or degrading treatment or punishment, under international human rights law."

Alabama had previously attempted to execute Smith — who had been on death row since 1989 — using a lethal injection in 2022 but this was called off at the last minute when authorities couldn't find an IV line.

'The world is watching'

Smith's lawyers had tried to block the nitrogen asphyxiation execution, arguing that the state was making him a test subject for an experimental execution method that could violate the constitutional ban on cruel and unusual punishment.

The legal battle went all the way to the Supreme Court, which ruled on Thursday that the execution could go ahead.

Justice Sonia Sotomayor was one of three dissenting judges who condemned the execution method.

"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," she wrote. "The world is watching."

Alabama Governor Kay Ivey said the execution was justice for the family of Elizabeth Sennett, who Smith was convicted of murdering in 1988.

"After more than 30 years and attempt after attempt to game the system, Mr Smith has answered for his horrendous crimes," he said. "I pray that Elizabeth Sennett's family can receive closure after all these years dealing with that great loss," she said in a statement.

UN and EU condemn execution

On Friday, United Nations chief Volker Turk said Smith's execution of by nitrogen gas suffocation could amount to torture. 

"I deeply regret the execution of Kenneth Eugene Smith in Alabama despite serious concerns this novel and untested method of suffocation by nitrogen gas may amount to torture, or cruel, inhuman or degrading treatment," he said in a statement. 

The European Union also said that it "deeply regrets" the execution.

"According to leading experts, this method is a particularly cruel and unusual punishment," a statement said. 

The EU has a blanket opposition to the death penalty and regular criticised executions carried out around the globe.

zc/sri (AP, AFP, Reuters, dpa)