Just hours before his scheduled execution, a US court has granted a temporary stay to a Missouri man due to his rare medical condition.

Russell Bucklew, 46, was scheduled to be put to death by lethal injection at 12:01am (1501 Wednesday AEST) for shooting to death the boyfriend of his ex-girlfriend and kidnapping and raping her in 1996.

In a 2-1 vote, a federal appeals court has issued the stay out of concern that Bucklew’s medical condition might cause “unnecessary pain and suffering beyond the constitutionally permissible amount inherent in all executions.”

Bucklew’s lawyers say he suffers from a condition that can cause weakened and malformed blood vessels which might result in severe pain during the execution.

The execution was to be the first in the United States since a botched execution on April 30 in Oklahoma.

In that case, the use of a mixture of three drugs was called into question when the procedure failed.

Although Derrell Lockett, 38, died from a heart attack, the three drugs administered did not work as intended, prison officials said.

Witnesses to Lockett’s execution described his death as gruesome, saying he grimaced and his body convulsed before prison authorities pulled a curtain so that witnesses could no longer see what was happening.

The governor of Oklahoma then ordered a review of the state’s execution procedures, while the planned execution of another Oklahoma inmate was postponed.

An execution earlier this year in Ohio also went badly.

Authorities in that state came under criticism for using a two-drug mixture for the first time.

In that execution, it took 25 minutes for Dennis McGuire, 53, to die.