U.S. President Donald Trump forcefully criticized Pope Leo late on Sunday in an unusual, direct attack on the leader of the 1.4-billion-member Catholic Church that drew immediate rebuke from believers.

The president, in an apparent response to the pope's growing criticisms of the U.S.-Israeli war on Iran and the Trump administration's hard-line immigration policies, said Leo was "terrible".

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

Catholics on social media quickly lambasted Trump for attacking the leader of their Church, who they believe is the successor of St. Peter, one of Jesus' 12 apostles.

"There is no ambiguity about the situation now," Massimo Faggioli, an expert on the papacy, told Reuters.

He compared the comments to efforts by the leaders of Germany and Italy during World War Two to draw the late Pope Pius XII to support their causes.

"Not even Hitler or Mussolini attacked the pope so directly and publicly," said Faggioli.

Archbishop Paul S. Coakley, president of the U.S. Conference of Catholic Bishops, said he was disheartened by Trump's comments.

"Pope Leo is not his rival; nor is the Pope a politician. He is the Vicar of Christ who speaks from the truth of the Gospel and for the care of souls,” he said in a statement.

POPE LEO VOCAL ON WAR AND IMMIGRATION ISSUES

Leo, originally from Chicago, is the first U.S. pope. Known for choosing his words carefully, he has ​emerged as an outspoken critic of the Iran war in recent weeks and decried the "madness of war" on in a peace appeal on Saturday.

Last year, he questioned whether the Trump administration's hard-line immigration policies were in line with the 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.

Trump wrote in his post Sunday that "Leo should get his act together as Pope", 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 U.S. 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. The pope is due to leave on Monday for an ambitious, 10-day tour of four countries in Africa.

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

The pope's call for 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 U.S. 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.

-Reporting by Kanishka Singh in Washington and Chris Kirkham in Los Angeles; Editing by Christopher Cushing, Sergio Non and Thomas Derpinghaus/Reuters