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

Michael Smith, 41, 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 Central Time (1509 GMT) and Smith was pronounced dead 11 minutes later. A "spiritual advisor" was present in the execution chamber at the time.

Smith was convicted in 2003 of the separate 2002 murders in Oklahoma City of Janet Moore and Sharath Babu Pulluru, a convenience store clerk.

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 and the US Supreme Court turned 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 United States this year including one in Alabama that was the first using nitrogen gas.

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

Another execution by lethal injection had been scheduled to take place in Idaho in February but was halted after a medical team was unable to insert an intravenous line.

Capital punishment has been abolished in 23 US states, while the governors of six others -- Arizona, California, Ohio, Oregon, Pennsylvania and Tennessee -- have put a hold on its use.

There were 24 executions in the United States in 2023, all of them carried out by lethal injection.