"Pope Leo is WEAK on Crime, and terrible for Foreign Policy," Trump wrote in a post on Truth Social.

Leo, who last year became the first US-born pope, has ​emerged as an outspoken critic of the US-Israeli war on Iran that began on February 28. He previously questioned the Trump administration's approach to immigration. 

"Leo should get his act together as Pope," Trump wrote in his post Sunday, later telling reporters he was "not a big fan" of the pontiff.

Trump's broadside against Leo also accused him of being "weak on nuclear weapons," several days after the pope said the US president's threat to destroy Iranian civilization was "truly unacceptable."

In a speech on Palm Sunday last month in St. Peter's Square in the Vatican, the pope said God rejects the prayers of leaders who start wars and have their "hands full of blood," calling the conflict in Iran "atrocious." 

Leo has also called on Trump to find an "off-ramp" to end the conflict and "decrease the amount of violence."

In his post, Trump suggested that Leo was only elected to lead the Catholic Church last year "because he was an American, and they thought that would be the best way to deal with President Donald J. Trump."

The Vatican did not immediately respond to a request for comment.

Leo has called for "deep reflection" about the way migrants are being treated in the US. 

Last year the pope also questioned whether Trump's hard-line immigration policies were in line with the Catholic Church's pro-life teachings.

"Someone who says I am against abortion but I am in agreement with the inhuman treatment of immigrants in the United States, I don't know if that's pro-life," the pontiff said in September.

The Vatican leader's belief in a more compassionate approach to immigration - a sentiment expressed by several of Leo's predecessors - stands in contrast to the stance of Trump, who has argued that the US must curtail immigration from developing countries to reduce crime. 

"He's a very liberal person and he's a man who doesn't believe in stopping crime," Trump told reporters on Sunday night.

Trump also had a rocky relationship with Leo's predecessor, Pope Francis, who criticized Trump's immigration policy proposals when he first ran for president and suggested Trump was "not a Christian." Trump had called Francis "disgraceful" in early 2016.

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