Arbour said: "International law, as it currently stands, only allows the imposition of the death penalty as an exceptional measure within rigorous legal constraints."
She said concerns that she expressed about the fairness and impartiality of Saddam's trial applied equally to the other two men, whose appeals against their sentences - like that of Saddam - have been rejected.
The UNHCHR chief's call was backed by the Ban Ki-Moon, the new UN secretary general, a spokeswoman for his office said.
Michele Montas said: "The secretary general strongly believes in the wisdom of Article 3 of the Universal Declaration of Human Rights which states everyone has the right to life, liberty and security of person.
"In that context, he fully endorses the call made today by Louise Arbour."
Under Iraq's international obligations, she said, the Baghdad government was bound to give the two men the opportunity to seek commutation of the sentence or pardon.
No second term
Asked whether he would accept a second term, al-Maliki said in an interview published on Tuesday by the Wall Street Journal: "Impossible."
In another development, the Iraqi prime minister, now under heavy criticism for his handling of Saddam's execution, has said he has no interest in a second term and wished he could be done before the end of his current term.
|Al-Maliki has said he will not seek a second|
term of office [AP]
"I didn't want to take this position. I only agreed because I thought it would serve the national interest, and I will not accept it again."
His term is intended to be four years, but it could be cut short by a power shift in parliament.