The execution of a US death row inmate has been delayed at the 11th hour after a mix-up with the drugs for the lethal injection.

Richard Glossip told reporters he was standing in a holding cell a few feet from the Oklahoma state death chamber when he received the news minutes before the scheduled injection.

Oklahoma governor Mary Fallin delayed the punishment after prison officials said one of the three drugs they had received to carry out the injection was the wrong one.

Attorney general Scott Pruitt's office said the reason for the mix-up was unclear, and the Corrections Department director refused to take questions.

A lawyer for Glossip said the incident shows more public oversight of executions is needed.

Glossip, who has long maintained his innocence, was set to die for the 1997 killing of an Oklahoma City motel owner.

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