The US Supreme Court has denied petitions to halt the execution of an Arkansas inmate, allowing the state to proceed with its first execution in 12 years after its top court approved the use of a drug in its lethal injection mix.

The state is preparing to execute Ledell Lee at its Cummins Unit in Grady, which houses the state's death chamber, but it is still possible that the killing will be blocked.

The US Court of Appeals for the 8th Circuit in St Louis, which is considering a last-minute request from Lee for DNA testing, has issued a stay until 9:15pm on Thursday (01:15 GMT Friday).

The death warrant for Lee expires at midnight.

Lawyers for Lee, 51, who has maintained his innocence for years, launched last-minute appeals to halt the execution with federal courts and the US Supreme Court

Lee was convicted and sentenced to death for beating Debra Reese to death with a tyre iron in 1993.

Reese's relatives were at the Cummins Unit prison and told local media Lee deserves to die for a crime that ripped their lives apart.

The Supreme Court ruling was the latest in a flurry of legal wrangling as the state seeks to carry out a series of executions before one of the drugs used in its lethal injection mix, the sedative midazolam, expires by the end of the month.

Lawyers for the inmates argued the state's rush to the death chamber amounted to cruel and unusual punishment, violated the inmates' right to counsel and their right to access the courts and counsel during the execution process.

The US Supreme Court denied the petitions. One of them was a 5-4 decision in which new Justice Neil Gorsuch sided with the four other conservative justices in denying the motion while the court's liberals dissented.

Earlier on Thursday, the top Arkansas court overturned a previous ruling that had blocked the use of one of the other three drugs the southeastern state planned to use.

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Arkansas had planned to execute eight inmates in 11 days, the most of any state in as short a period since the US Supreme Court reinstated the death penalty in 1976.

Courts have halted four of those executions. The state's plan and the legal battles have raised questions about US death chamber protocols and lethal injection drug mixes. Back-to-back executions set for Monday were indefinitely halted.

Previous legal challenges prevented the state from executing any prisoner since 2005.

In the ruling on the state's lethal injection drug, the Arkansas Supreme Court agreed with the state that it did nothing illegal in acquiring the muscle relaxant vecuronium bromide and lifted an order by a state circuit judge on Wednesday that blocked its use.

US pharmaceutical wholesaler McKesson Medical-Surgical Inc had argued that it sold Arkansas the drug for medical use, not executions, and that it would suffer harm financially and to its reputation if the executions were carried out.

Arkansas had also planned to execute convicted murderer Stacey Johnson on Thursday. But the Arkansas Supreme Court on Wednesday issued a halt to Johnson's execution after he requested DNA testing he said could prove his innocence.

The attorney general's office said on Thursday it would not appeal the decision, meaning his planned execution was off. Johnson was convicted of the 1993 murder and sexual assault of Carol Heath.

The state's protocol calls for use of midazolam to render the inmate unconscious, vecuronium bromide to stop breathing and potassium chloride to stop the heart..

Anti-death penalty activists have protested in Arkansas against what has been labelled "assembly line killings".

Midazolam has been linked to botched executions, and critics say it has been proven ineffective in rendering unconsciousness prior to administration of the two lethal agents.

The American Civil Liberties Union (ACLU) said in a petition that "Arkansas wants to use an execution drug combination - with midazolam - that's never been used before in the state and that risks making prisoners feel as if they are burning alive from the inside while paralysed".