WASHINGTON — Alabama, one of the US states carrying out the greatest number of executions, has suspended its use of the rapidly dwindling sedative sodium thiopental in favor of another death penalty drug.

"Due to the national shortage of sodium thiopental, the Alabama Department of Corrections has amended its execution protocol to allow for the use of pentobarbital for future executions," said Brian Corbett, spokesman for the state prison system, in a statement obtained by AFP Wednesday.

Alabama now becomes the fourth US state to replace the powerful sedative, which forms part of a three-drug cocktail for lethal injections with pentobarbital, an anesthetic commonly used to euthanize animals.

Sodium thiopental is no longer manufactured in the United States and existing stocks are expiring.

Three other states, Georgia, Kentucky and Tennessee, also have stopped using sodium thiopental.

Corbett said that prior to ending its use of sodium thiopental, Alabama had acquired "a small amount" of the drug from Tennessee, but willingly turned it over to federal authorities after being asked to do so by the US Drug Enforcement Administration.

Bryan Stevenson, director of the Equal Justice Initiative, which engages in advocacy on behalf of prison inmates, said that Alabama's actions reflect the lack of clarity about the current and future death penalty protocols across the country.

"The reliability and legality of the death penalty requires clear and carefully documented execution protocols which the state of Alabama has not developed," Stevenson said.

"The Alabama Department of Corrections should not be allowed to make up procedures for carrying out executions without accountability or transparency and in a manner that suggests some deception.

This raises questions about the legality and the propriety of these scheduled executions," he said.

He added that corrections officials in the state have not proved that they are trained in administering the new drug, and said that the courts so far have not approved a protocol for the use of new execution drugs.

"In the absence of administrative oversight, the court needs more information than the state has provided regarding their plans for carrying out these executions," said Stevenson.

The non-profit Death Penalty Information Center said this month that Alabama ranked second only to Texas in 2010 and 2011 for the number of executions carried out, and sixth overall on a list of executions by state since 1976.