US President Donald Trump on Thursday said Pope Leo was free to say what he wanted but that it was important for him to understand that Iran could never have a nuclear weapon.
Trump has repeatedly criticised the religious leader, who has emerged as an outspoken critic of the US-Israeli war on Iran in recent weeks.
Trump says he is responsible for Leo's appointment, and has accused him of being "weak" on foreign policy.
"The Pope has to understand - it's very simple - Iran cannot have a nuclear weapon. The world would be in great danger," he told reporters at the White House.
Trump's criticism has triggered widespread backlash from US Christians across the political spectrum.
Leo, originally from Chicago, decried the "madness of war" in a peace appeal on Saturday.
The US and Israel attacked Iran on 28th February. Iran responded with its own strikes on Israel and Gulf states with US bases. US-Israeli strikes on Iran and Israeli attacks in Lebanon have killed thousands and displaced millions.
Leo told Reuters on Monday he would keep criticising the war, regardless of Trump's comments.
"The Pope can say what he wants, and I want him to say what he wants, but I can disagree. I think that Iran cannot have a nuclear weapon," Trump said.