U.S. Secretary of State Marco Rubio will meet Pope Leo at the Vatican on Thursday, in a potentially fraught encounter as President Donald Trump has continued a series of disparaging attacks on the Catholic leader over the Iran war.

Rubio, who also serves as Trump's national security adviser, was due to arrive at the Vatican's Apostolic Palace around 11:15 a.m. (0915 GMT) for the visit, the first between the pope and a Trump cabinet official in nearly a year.

The closed-door meeting is expected to last about a half-hour, after which Rubio will also meet the Vatican's top diplomat, Italian Cardinal Pietro Parolin.

Leo, the first U.S. pope, drew Trump's ire after becoming a firm critic of the U.S.-Israeli war on Iran and the Trump administration's hardline anti-immigration policies.

The president has kept up an unprecedented series of public attacks on the pope in recent weeks, drawing a backlash from Christian leaders across the political spectrum.

On Monday, Trump falsely suggested the pope believed it was okay for Iran to obtain nuclear weapons and said Leo was "endangering a lot of Catholics" by opposing the war.

Leo told journalists after the latest attack that he was spreading the Christian message of peace. The pope also firmly rejected the idea that he supported nuclear weapons, which the Catholic Church teaches are ​immoral.

"The mission of the Church is to preach the Gospel, to preach peace," said the pope. "The Church has spoken out for years against ​all nuclear arms, on that there is ⁠no doubt."

US AMBASSADOR EXPECTS 'FRANK' CONVERSATION

Leo, who on Friday marks his first year leading the 1.4-billion-member Church, has grown more outspoken on the world stage in recent weeks.

During a four-nation African tour last month he forcefully decried the direction of global leadership and said the world was "being ravaged by a handful of tyrants", in comments he later said were not aimed directly at Trump.

Rubio is Catholic, as is Vice President JD Vance. The two met Leo a year ago after attending the pope's inaugural mass.

Rubio said at a White House briefing on Tuesday that he expected to discuss Cuba and concerns over religious freedom around the world with Leo.

He was flying to Rome without any press accompanying him on his plane, which is unusual for a U.S. secretary of state.

The U.S. ambassador to the Holy See, Brian Burch, told journalists earlier on Tuesday that the conversation between the pope and cabinet official was likely to be "frank".

Rubio is visiting Rome for two days. He is due to meet Italian Prime Minister Giorgia Meloni, who has defended the pope from Trump, on Friday. Meloni's defence minister has also said the war in Iran puts U.S. leadership at risk.

-Reporting by Joshua McElwee; Editing by Alex Richardson/Reuters