he US Supreme Court granted a last minute stay to a convicted murderer who is challenging the drugs prison officials had planned to use for his execution on Wednesday.

Death Row inmate Herbert Smulls was to be executed overnight Wednesday by the state of Missouri, but Justice Samuel Alito late Tuesday granted him a reprieve, amid ongoing controversy over the use of lethal pentobarbital manufactured by a compounding pharmacy whose identity has not been revealed.

Alito, the justice assigned to cases from the region of the United States that includes Missouri, is expected to rule soon on a similar petition from Louisiana inmate Christopher Sepulvado, due to be put to death next week.

Sepulvado was condemned for fatally beating and scalding his six-year-old stepson in 1992.

Smulls is on Death Row for killing a jeweler during an armed robbery. The governor of Missouri overnight rejected his petition for a stay of execution.

Attorneys for both men maintain that the failure to reveal the identity of the pharmacy makes it impossible to know if the execution would constitute "cruel and inhumane punishment" under the Eighth Amendment of the US Constitution.

They suggested that the barbiturate used to put the inmates to death might have been tested and approved by the same lab that tested drugs used in a controversial execution carried out recently in Oklahoma.

Oklahoma Death Row inmate Michael Lee Wilson, executed on January 9, complained that he could feel his whole body "burning" as he was put to death.

An expert cited by lawyers for Smulls said the same Oklahoma lab is believed to have approved drugs produced in Massachusetts responsible for an outbreak of meningitis in November 2012.