Chicago: The US state of Tennessee used a controversial lethal injection procedure on Thursday to execute a man who was convicted of raping and killing a child after the nation's top court declined his final bid for a stay.

"I just want to say I'm really sorry. And that ... that's it," Billy Ray Irick said in his final words before prison officials in Nashville, Tennessee. He was pronounced dead at 7:48 pm local time, officials said at a press conference. Irick was the first inmate to be executed in Tennessee since 2009. 

The US Supreme Court had earlier denied a stay of execution for the convict, rejecting concerns about the inmate potentially feeling sensations equivalent to being ‘burned alive’. The high court's decision was countered with a blistering dissent from Justice Sonia Sotomayor, who questioned whether Billy Ray Irick's planned execution would be too painful and whether allowing it to proceed required accepting ‘barbarism’.

The 59-year-old was convicted in 1986 of raping and killing seven-year-old Paula Dyer. His lawyers have argued that Irick has a history of severe mental illness.

In an appeal to the Supreme Court, Irick's lawyers challenged Tennessee's lethal injection protocol, which included the use of the sedative midazolam. Other states have also used the drug for executions with mixed results.

Sotomayor, citing lower court testimony about the potential risks of midazolam executions, dissented.

Medical experts warned the drug may not be strong enough to keep a prisoner unconscious once he starts to feel pain.

If Irick were to awaken, a paralytic used as the second drug in Tennessee's lethal injection would prevent him from alerting officials that he can feel pain, Sotomayor noted.

"Medical experts explained in painstaking detail how the three-drug cocktail Tennessee plans to inject into Irick's veins will cause him to experience sensations of drowning, suffocating, and being burned alive from the inside out," Sotomayor wrote. "If the law permits this execution to go forward in spite of the horrific final minutes that Irick may well experience, then we have stopped being a civilized nation and accepted barbarism," he added.

Three out of 14 executions in the US this year have employed midazolam, according to the Death Penalty Information Centre. But the drug has been the subject of multiple legal challenges.

Ohio halted executions in 2014 after the botched execution of Dennis McGuire using midazolam. The prisoner appeared to suffer for several minutes before dying.

A federal appeals court in 2017 granted Ohio permission to resume executions after officials increased the dose of the drug by a factor of 50.

Meanwhile, the state of Nevada last month postponed an execution after midazolam-manufacturer Alvogen successfully sued to stop its product from being used in executions.