GENEVA - United Nations experts on Wednesday called on U.S. authorities not to go ahead with the planned execution of an inmate by nitrogen hypoxia, saying the method may subject him to "cruel, inhuman or degrading treatment or even torture".

Kenneth Smith, convicted for a murder-for-hire committed in 1988, is scheduled to be executed in the U.S. state of Alabama on Jan. 25 using the method, which is intended to deprive him of oxygen by using a face mask connected to a cylinder of nitrogen.

Smith, 58, is one of only two people alive in the U.S. to have survived an execution attempt after Alabama botched his previously scheduled execution by lethal injection in November 2022 when multiple attempts to insert an intravenous line into a vein failed.

"This will be the first attempt at nitrogen hypoxia execution," four U.N. Special Rapporteurs said in a statement, saying the method could cause "grave suffering" and likely be at odds with the prohibition on torture and other cruel, inhuman or degrading punishment.

"We are concerned that nitrogen hypoxia would result in a painful and humiliating death."

Smith's lawyers have said the untested gassing protocol may violate the U.S. Constitution's ban on "cruel and unusual punishments", and have argued a second attempt to execute him by any method is unconstitutional.

Most U.S. executions are carried out using lethal doses of a barbiturate, but some states have struggled to obtain the drugs because of a European Union law banning pharmaceutical companies from selling drugs that can be used in executions to prisons. REUTERS