In his first encyclical, a sort of manifesto, US-born Leo focused mainly on the rise of artificial intelligence but also returned to an anti-war message that has caused tensions with Washington.

"Today, more than ever, without prejudice to the right to self-defense in the strictest sense, it is important to reaffirm that the 'just war' theory, which has all too often been used to justify any kind of war, is now outdated," he wrote in Magnifica Humanitas (Magnificent Humanity).

"Humanity possesses far more effective and capable tools for promoting human life and resolving conflicts, such as dialogue, diplomacy and forgiveness," he wrote.

The pope has been a high-profile critic of the Middle East war which started with US and Israeli air strikes on Iran in February.

He condemned Trump's threat to destroy Iran as "truly unacceptable" and urged Americans to demand that US lawmakers "work for peace".

Trump slammed the pontiff as "WEAK on Crime, and terrible for Foreign Policy" -- to which Leo replied that he had a "moral duty to speak out".

The Trump administration has repeatedly justified its attack on Iran as a way to prevent the country from developing a nuclear weapons capability.

US Vice President JD Vance, a Catholic convert, has used the "just war" argument and has urged the pope to be "careful" on theological matters.

In an interview earlier this month, Trump again criticised the pope, alleging that Leo believes it is "OK for Iran to have a nuclear weapon".

"I think he's endangering a lot of Catholics and a lot of people," Trump said.

Asked about the comments, Leo said that the Catholic Church's mission was to "preach peace" and the Gospel.

"If anyone wishes to criticise me for proclaiming the Gospel, let them do so truthfully," he told reporters.

US Secretary of State Marco Rubio met with the pope earlier this month in an attempt to repair relations.