The Supreme Court has refused to suspend the execution of a death row inmate in Missouri, despite the convict's lawyers arguing that a compound of lethal injection drugs used would be unregulated and could result in undue suffering.

William Rousan, 57, was executed on Wednesday after being convicted for the 1993 murders of Grace Lewis, 62, and her husband Charles, 67, in a plot to steal cattle from their farm, AP reported.

In their petition to the US Supreme Court, Rousan's attorneys had said the state was planning to use "compounded pentobarbital prepared by an unknown person in an unknown manner, without any assurance by an accredited laboratory that the substance is what the state purports it to be".

Rousan's lawyers argued that he had a right to know what he will be injected with.

But the court rejected the petition and refused to stop the execution.

Eric Butts, one of the attorneys, said there were no more possibilities for appeal. "This is it. They [the Supreme Court] just are really not interested in the situation."

The lethal injection was carried out a few days after the scheduled executions of two inmates were halted in Oklahoma, as defence lawyers had argued the prisoners had the right to challenge the secrecy over the drugs used.

Lawyers for death row inmates in several US states have argued that drugs obtained for lethal injections from compounding pharmacies could lead to unnecessary suffering, which is considered as a cruel and unusual punishment in violation of the US Constitution.

Prison officials have rejected these arguments and have been refusing to reveal where they get the substances from.

Louisiana and Ohio, however, have seen executions delayed this year because of concerns about suffering that might be caused by untraditional drug supplies, Reuters said.

Capital punishment is legal in 32 states in the US. Out of the 3,170 prisoners on death row, 43 executions were carried out in 2012.

Three quarters of all executions occurred in only four states: Texas, Arizona, Mississippi and Oklahoma.