Sept 22 (Reuters) - The Supreme Court of Alabama is weighing whether to allow the state to become the first to execute a prisoner with a novel method: asphyxiation using nitrogen gas.

Last month, Alabama Attorney General Steve Marshall asked the court to allow the state to proceed with gassing Kenneth Smith, who was convicted of murder in 1996, using a face mask connected to a cylinder of nitrogen intended to deprive him of oxygen.

Smith's lawyers have said the untested 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.

In a reply brief filed with the court on Friday, they called the nitrogen gas protocol "so heavily redacted that it is unintelligible," and said Smith had not yet exhausted his appeals.