McAlester, Oklahoma - A man convicted of killing two people more than two decades ago was executed by lethal injection in Oklahoma on Thursday, officials said.

Michael Smith was put to death at the Oklahoma State Penitentiary in McAlester, the Department of Corrections officials said in a statement.

It said the execution process began at 10:09 AM CT and Smith was pronounced dead 11 minutes later. A "spiritual advisor" was present in the execution chamber at the time.

The 41-year-old was convicted in 2003 of the separate 2002 murders in Oklahoma City of Janet Moore and Sharath Babu Pulluru, a convenience store clerk.

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According to the Death Penalty Information Center, Smith initially confessed to shooting Moore and Pulluru, but later claimed he had been on drugs and could not remember being arrested.

The Oklahoma Pardon and Parole Board denied Smith's request for clemency last month, with the US Supreme Court turning down his last-ditch appeal for a stay of execution on Thursday.

Smith's attorneys had argued that his life should be spared because he is intellectually disabled and he abused drugs for years.

There have been three other executions in the US this year, including a hugely controversial one in Alabama that was the first using nitrogen gas.

The two others – in Georgia and Texas – were carried out by lethal injection.