Donald Trump has launched an unusual and scathing attack on Pope Leo over his opposition to US immigration policy and the war in Iran.

The US president accused the pontiff of being 'WEAK on Crime and terrible for Foreign Policy' in a Truth Social post, later telling reporters he was 'not a big fan'.

Pope Leo has been a staunch critic of the war, calling Trump's threat to destroy Iranian civilization 'unacceptable' and calling for him to find an 'off-ramp' to end the conflict.

He has also questioned whether Trump's hard-line immigration policy was in line with the pro-life stance of the Catholic Church.

Trump's remarks came as the pontiff embarked on an 11-day trip to Africa, his second major foreign trip since being elected last year.

The US president wrote in Sunday's post that Pope Leo 'should get his act together' and said he was 'weak on nuclear weapons', apparently referring to Tehran's attempts to develop nuclear capabilities, which is cited as one of the reasons for the US and Israel engaging in military operations against Iran.

He also suggested that the pontiff was elected 'because he was American, and they thought that would be the best way to deal with President Donald J Trump'.

'If I wasn't in the White House, Leo wouldn't be in the Vatican.'

Asked by reporters later to clarify the post, he said: 'I don't think he's doing a very good job; he likes crime, I guess.'

Trump added: 'He's a very liberal person, and he's a man who doesn't believe in stopping crime; he's a man who doesn't believe we should be toying with a country that wants a nuclear weapon so they can blow up the world.'

The remarks drew immediate criticism from Catholics, with one expert drawing a parallel between Trump's comments and the attitudes of fascist dictators during World War II. Massimo Faggioli asserted, 'Not even Hitler or Mussolini attacked the Pope so directly and publicly.'

The Pope has consistently used public addresses to denounce global conflicts and advocate for de-escalation in the Middle East. When Trump threatened Iran, claiming 'a whole civilization will die tonight', the Pope responded that such threats were 'truly unacceptable'.

In his Easter address, the pontiff stated that people were becoming accustomed to violence and indifferent to thousands of deaths, urging that 'Let those who have weapons lay them down. Let those who have the power to unleash wars choose peace.'