The US Supreme Court on Wednesday declined to halt Alabama from proceeding with the nation's first execution using nitrogen gas to carry out the death penalty on convicted murderer Kenneth Smith, who survived a botched lethal injection in 2022 that helped prompt a review of the state's death penalty procedures.

The justices denied Smith's request to stay his execution, which is scheduled for Thursday, and declined to hear his legal challenge contending that a second execution attempt by Alabama – after the first failed attempt caused him severe trauma – would violate the US Constitution's Eighth Amendment protections against cruel and unusual punishment.

No justice publicly dissented from the decision.

Smith, 58, is separately contesting the legality of Alabama's nitrogen gas protocol on Eighth Amendment and other grounds.

That litigation still could come to the Supreme Court, potentially giving the justices another opportunity to decide whether to halt the execution. A judge ruled against Smith concerning the protocol on January 10. The Atlanta-based 11th US Circuit Court of Appeals has yet to rule on Smith's appeal.

Smith's attorney Robert Grass declined to comment.

A majority of the Supreme Court's justices in 2022 cleared the way for the first attempted execution of Smith, who was sentenced to death for his role in a 1988 murder-for-hire plot. The nine-member court's three liberal justices dissented from that previous decision.

Alabama's gassing method – called nitrogen hypoxia – was designed to deprive Smith of oxygen by placing a mask connected to a cylinder of nitrogen over his face.

Smith's botched execution was the third consecutive instance in which Alabama officials encountered problems or delays inserting intravenous lines for a scheduled lethal injection, with two of the executions, including Smith's, eventually called off, according to court filings.

The problems prompted Alabama Governor Kay Ivey, a Republican, to announce a review of the state's execution procedures. Officials completed the review a few months later, saying they obtained new equipment and would add to the pool of available medical personnel for executions.

Alabama Republican Attorney General Steve Marshall in a filing called the method "perhaps the most humane method of execution ever devised."

Ivey set Smith's execution to take place within a 30-hour time frame beginning at 12am on Thursday and expiring at 6am the following day. (Reuters)