Washington, Sep 27 (IANS): An inmate of a prison in the Alabama state of the US who is scheduled to be sentenced to death later on Thursday will become the fifth execution in the country this week and the 18th this year.

Alan Eugene Miller, 59, will be executed with nitrogen gas, which is only the second time this method will have been used in the US ever.

Miller is on death row for killing three men in a workplace shooting in 1999.

Earlier on Thursday, an inmate of a prison in Oklahoma State was put to death with a lethal injection.

Emmanuel Littlejohn, 52, was executed for the murder of a man during a convenience store robbery in 1992. He had maintained his innocence to the last claiming he was present during the robbery but did not shoot the victim.

Miller will enter an execution chamber for the second time on Thursday evening. The first time was in September 2022.

A lethal injection was to be used then but the staff called it off after failing to find a vein for more than an hour.

In January, Alabama became the first state in the country, and the first anywhere in the world, to use nitrogen gas to execute an inmate sentenced to death.

The prisoner, Kenneth Smith, shook violently as the nitrogen gas was administered and then writhed before his body eventually stopped moving, the New York Times reported citing witnesses.

Advocates of the nitrogen hypoxia method, which replaces oxygen inhaled by an inmate with pure nitrogen through a mask, have argued that a person would likely lose consciousness shortly into the procedure, making it more humane than other execution methods.

"Clearly it was not the instant, painless death that they promised," Jonathan Groner, a professor of surgery at The Ohio State University College of Medicine, told CNN last about Smith's execution.

"There's a lot of suggestions that it wasn't good, or that it wasn't pleasant."