"The use of nitrogen gas -- it is troubling to us," White House spokeswoman Karine Jean-Pierre told reporters. "We are deeply troubled by it."

On Thursday, the southern state of Alabama executed Kenneth Smith, a 58-year-old convicted murderer, by pumping nitrogen gas into a facemask, causing him to asphyxiate. Alabama Attorney General Steve Marshall reportedly defended the decision to put Smith to death through nitrogen asphyxiation saying the execution had been carried out in a "professional manner."

As per local reports, as soon as the nitrogen gas was administered, Smith began writhing and thrashing for approximately two to four minutes, followed by around five minutes of heavy breathing.

Recent executions in the United States have been carried out by lethal injection but Alabama and two other states, Oklahoma and Mississippi, have authorized the use of nitrogen gas.

EU, UN Human Rights criticise Nitrogen asphyxiation

The experimental method of the death penalty which violated the right to life and caused an inhuman extent of suffering was hugely criticised by the 27-nation European Union and the Geneva-based UN rights office.

“He was writhing and clearly suffering," Ravina Shamdasani, spokesperson for the UN Human Rights Office, said at a regular UN briefing in Geneva. "Rather than looking for novel, untested methods to execute people, let's just bring an end to the death penalty. This is an anachronism that doesn't belong in the 21st century.”

She said the UN human rights chief, Volker Türk, had written to authorities in Alabama about the issue, and said his office will continue to speak out and use “every tool in our toolbox" to prevent other states from doing so.

It was the first time a new execution method has been used in the US since 1982, when lethal injections were introduced and later became the most common method.

“According to leading experts, this method is a particularly cruel and unusual punishment,” the diplomatic service of the EU said in a statement. It also expressed concern that the number of executions in the US increased last year.

“Twenty-four people were executed in five states despite a steady, overall decline of the use of capital punishment in the US since 2020," it said. “We call for states that maintain the death penalty to implement a moratorium and move towards abolition, in line with the worldwide trend.”