Geneva, Switzerland - United Nations experts called Wednesday for the halt to an imminent execution using nitrogen gas in Alabama, warning the rare method could amount to "torture".

Thursday's scheduled execution of Carey Grayson over his involvement in a 1994 murder of a hitchhiker would be the third time nitrogen gas has been used to carry out the death penalty in the US.

The method, which has been harshly condemned by the UN, was used for the first time in January and then again in September, both times in Alabama.

The execution is carried out by pumping nitrogen gas into a facemask, causing the prisoner to suffocate.

Media reports of the first nitrogen gas execution in Alabama this year said the prisoner writhed about for several minutes before dying.

The inmate in the second case was reported to have struggled in his restraints for two minutes as he suffocated.

"This method may amount to cruel, inhuman or degrading treatment, or even torture," said a statement from eight independent UN rights experts, urging US and Alabama authorities "to halt Grayson's execution and any others scheduled to be executed in this manner".

"We reiterate our call for an urgent ban on execution by nitrogen asphyxiation, which is clearly prohibited under international law," said the experts, including the UN special rapporteurs on extrajudicial executions and torture.

They reminded the US of its international obligations as a party to the International Covenant on Civil and Political Rights and the Convention against Cruel, Inhuman or Degrading Treatment or Punishment.

The experts, who were appointed by the UN Human Rights Council but who do not speak on behalf of the United Nations, also voiced concern that defendants in the US challenging their execution method as potentially inhumane or degrading treatment were asked to prove there was an alternative method of execution available.

"We emphasize that the prohibition on torture or to cruel, inhuman or degrading treatment or punishment is absolute, never acceptable and not dependant on alternatives," they said.

The experts also voiced grave concern that other US states were taking steps to use nitrogen gas for executions.

More broadly, they called on the US to "join a growing global consensus towards universal abolition of the death penalty, beginning by promptly imposing a moratorium on executions".