US President Donald Trump said it was important for him for Pope Leo to understand that Iran cannot have nuclear weapons, Reuters reported.
"The Pope needs to understand - it's very simple - Iran cannot have nuclear weapons. The world would be in great danger," he told reporters at the White House.
Trump has repeatedly criticized the religious leader, who has emerged as an outspoken critic of the US-Israeli war against Iran in recent weeks.
Trump said he was responsible for appointing Leo and accused him of being "weak" in foreign policy.
Trump's criticism has drawn widespread backlash from American Christians across the political spectrum.
Lew, a native of Chicago, condemned the "madness of war" in a peace appeal on Saturday.
The United States and Israel attacked Iran on February 28. Iran responded with its own strikes on Israel and Gulf states with U.S. bases. The U.S.-Israeli strikes on Iran and the Israeli attacks in Lebanon have killed thousands and displaced millions.
Lew told Reuters on Monday that he would continue to criticize the war, regardless of Trump's comments.
"The pope can say whatever he wants and I want him to say whatever he wants, but I can disagree. I don't think Iran can have a nuclear weapon," Trump said.