The priest was among several people who witnessed the execution.
Convicted murderer Kenneth Eugene Smith became the first man in the US to be executed with nitrogen gas, a controversial method that was condemned by human right activists as well as the United Nations and White House.
The 22-minute execution that saw the 58-year-old murderer writhing and thrashing for several minutes was described as a "horror show" by his spiritual advisor Reverend Jeff Hood. The priest who was among several people who witnessed the execution said even the prison staff were unable to hide their shock over the scene.
“There was clear shock and surprise on their faces. In that circumstance it's hard to know what is what, but I know what I saw in terms of the horror on the faces of people in front of me," Hood told The New York Post, adding that he heard audible gasps from people as Smith struggled to breathe.
It looked like a scene produced for Hollywood, he said. "It's a scene that will never leave me."
The nitrogen hypoxia execution, a method described as "untested human experiment" by human rights activists, was carried out at Alabama's Holman Prison. It marked the first change from the usual method of lethal injection in the US. The method forces the prisoner to breathe nitrogen instead of oxygen through a respirator mask.
Hood claimed that while the prison authorities assured that the method would result in instant death, the actual scene of the execution could not be far from it.
“They said all along that this was going to be nearly instantaneous. They were told this is going to be quick, easy and painless. They kept saying this is the most humane way society has ever figured out how to execute people. That he would be gone, unconscious, in seconds. What we saw last night was minutes, minutes, and minutes of a horror show," he said, adding that Smith looked like a "fish out of water, flapping over and over again."
The White House on Saturday said it was "deeply troubled" by the execution. UN human rights chief Volker Turk, the EU and civil liberties groups also expressed concern about the manner of Smith's execution.
Kenneth Eugene Smith was on death row for more than three decades after being convicted of the 1988 murder-for-hire of a pastor's wife.