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

In a post on Truth Social, Trump accused the pontiff of being "WEAK on Crime and terrible for Foreign Policy", later telling reporters he was "not a big fan".

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

Trump's remarks came as Pope Leo embarked on an 11-day trip to Africa, his second major foreign trip since his election last year.

In Sunday's post, Trump wrote that Pope Leo "should get his act together" and said he was "weak on nuclear weapons", apparently referring to Tehran's attempts to become a nuclear power, cited as one of the reasons for the US and Israel going to war with Iran.

He also suggested that Leo 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 explain the post, he said: "I don't think he's doing a very good job, he likes crime, I guess."

"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," he added.

The remarks drew immediate criticism from Catholics, with one expert comparing the comments to the Pope's relationship with fascist dictators in World War Two.

"Not even Hitler or Mussolini attacked the Pope so directly and publicly," said Massimo Faggioli, quoted by Reuters.

Pope Leo has used numerous public addresses to denounce global conflicts and urge de-escalation in the Middle East.

When Trump threatened Iran, saying that "a whole civilisation will die tonight", he responded by saying the statement was "truly unacceptable".

In his Easter address, the pontiff said without mentioning any countries directly that people were growing accustomed to violence and indifferent to thousands of deaths.

"Let those who have weapons lay them down. Let those who have the power to unleash wars choose peace," he said.