A US prison has botched the execution of one death row inmate, delaying the execution of a second prisoner at the same facility.

Clayton Lockett, who along with fellow prisoner Charles Warner mounted a legal challenge regarding the cocktail of medicines to be used to kill them, died of a heart attack in Oklahoma on Tuesday.

The director of the Oklahoma Department of Corrections, Robert Patton, said Lockett's execution was halted about 20 minutes after the first of three drugs was administered. He said there was a vein failure.

Associated Press reporter, Bailey Elise McBride, was inside Oklahoma State Penitentiary where the executions were taking place.

She recounted the sequence of events on her Twitter feed: "He [Lockett] was conscious and blinking, licking his lips even after the process began. He then began to seize. At 6:33 the doctor said Lockett was unconscious and then at 6:34 Lockett began to nod, mumble move body."

She cited Patton, who addressed the media, in later tweets: "Sedated 7 minutes into execution, at that time began pushing 2nd and 3rd drugs. Some concern drugs were not having an effect. 7:06 inmate Clayton Lockett suffered heart attack and died.

"Prison Director has stayed execution for Charles Warner for 14 days. Lockett's vein blew during the execution preventing the chemicals from effectively entering his body."

Lethal dosage review

Lockett and Warner had sued the state for refusing to reveal details about the execution drugs, including where it had obtained them, but Oklahoma's Supreme Court ruled last week that the two death row inmates were not entitled to know the source of the drugs.

Following Tuesday's events, Governor Mary Fallin ordered an evaluation of "lethal dosage protocol".

In a statement posted to her website she said: "I have asked the Department of Corrections to conduct a full review of Oklahoma’s execution procedures to determine what happened and why during this evening’s execution of Clayton Derrell Lockett. I have issued an executive order delaying the execution of Charles Frederick Warner for 14 days to allow for that review to be completed.”

Lockett, 38, was convicted of shooting a 19-year-old in 1999 and watching as two accomplices buried her alive. Warner, 46, was convicted of raping and killing his roommate's 11-month-old daughter in 1997.

It would have been the first double execution in the US for 14 years and the first double execution in Oklahoma in 80 years.