Donald Trump's remarks about Pope Leo XIV were "unacceptable", Italian Prime Minister Giorgia Meloni has said.

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

"The Pope is the head of the Catholic Church, and it is right and normal for him to call for peace and to condemn every form of war," Meloni said in a statement.

Meloni, who is a Catholic and heads a right-wing coalition government, is a close ally of Trump and had so far been reluctant to condemn the US president's harsh criticism of Pope Leo.

Italian opposition parties have criticised Meloni for failing to speak out promptly.

Her coalition partner Matteo Salvini, the leader of the populist League party, said that "attacking the Pope... doesn't seem like a useful or intelligent thing to do."

After Trump's remarks, the Pope told reporters en route to Algeria that he did not want to get into a debate with Trump but would continue to promote peace.

Pope Leo has said he has "no fear" of the Trump administration and will continue to speak out against war after the US president launched an unusual and scathing attack over his stance on the Iran conflict.

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

On Monday, Trump doubled down on his criticism, saying he would not apologise to the "very weak" Pope.

In general, it is rare for a pope to directly address statements by world leaders.

There are more than 70 million Catholics in the US, about 20% of the population. They include Trump's Vice-President JD Vance.

Trump's comments 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 the Pope "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 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 to explain the post, he later 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."

In response, the Pope told reporters on board his plane to Algiers that he did not see his role as that of a politician but as one of spreading the message of peace.

"I have no fear of the Trump administration, or speaking out loudly of the message of the gospel, which is what I believe I am here to do, what the Church is here to do," he told reporters.

"I don't want to get into a debate with [Trump]," he added.

"Too many people are suffering in the world today. Too many innocent people are being killed. And I think someone has to stand up and say: there's a better way to do this."

The pontiff later used a speech to criticise "continuous violations of international law and neocolonial tendencies", urging leaders to embrace principles of justice and solidarity.

Trump's remarks also drew criticism from Catholics around the world, 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 prominent Italian Catholic commentator Massimo Faggioli.

The Pope 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".

The Pope has also criticised Trump's hard-line immigration policy, questioning whether it was possible for someone to be "pro-life" - a term normally associated with opponents of abortion - if they agreed with what he described as the "inhuman treatment of immigrants".

Pope Leo is seen as continuing the humanitarian tradition of his predecessor Pope Francis, who said Trump was "not Christian" during the 2016 election campaign because of his anti-immigrant language. Trump described the late Pope as "disgraceful".