He also writes on Truth Social: "I don't want a Pope who thinks it's okay for Iran to have a nuclear weapon."
The criticism comes after the Pope, who is also American, called for an end to the violence.
"Enough with the worship of self and money! Enough with demonstrations of power! Enough with war," the Pope said on Sunday.
The Pope said on Monday that he was not afraid of the Trump administration.
"I will continue with what I believe is the mission of the church in the world today," he told AP.
He emphasized that his general appeal for peace was not an attack on Trump or anyone else.
