"Today, as we all know, there was this threat against the entire people of Iran, and this is truly unacceptable," said the Pope, who has emerged as an outspoken critic of the Iran war.
"There are certainly issues here of international law, but even more than that, it is a moral question for the good of the (world's) people," he said.
In some of his strongest comments yet against the war, the pontiff urged people of good will to contact their political leaders and congressional representatives to demand they reject war and work for peace.
It is rare for the pope, who leads 1.4 billion Catholics around the world, to respond directly to a world leader.
Leo spoke to reporters as he left his country house in Castel Gandolfo, south of Rome.
The pontiff also said that many people have referred to the conflict as an "unjust war," using terminology to indicate hostilities that go against the Catholic Church's strong pro-life teachings.
He urged people to "remember especially the innocent children, the elderly, the sick, so many people who have already become or will become victims of this continued warfare".
with Reuters