The task was not easy, given President Donald Trump's recent sharp criticism of both the Catholic leader and Italy's far-right Prime Minister Giorgia Meloni, a close ally of the US leader.

"The world has to start asking itself, what is it willing to do if Iran tries to normalise a control of an international waterway? I think that's unacceptable," he told reporters after meeting Meloni.

The appeal was aimed at Italy as well as other European countries, which Trump criticised for not helping the United States to protect the Strait.

Tehran seized control of the narrow chokepoint to the Gulf, a major transport route for oil, gas and fertiliser, after US and Israeli forces attacked Iran on February 28, triggering the Middle East war.

After saying 5,000 troops will be withdrawan from Germany, Trump has threatened to pull US troops from Italy and Spain due to their refusal to get involved in the conflict, and has questioned his country's membership in NATO.

"If one of the main reasons why the US is in NATO is the ability to have forces deployed in Europe that we could project to other contingencies, and now that's no longer the case, at least when it comes to some NATO members, that's a problem, and it has to be examined," Rubio said.

He added, however that the US president had not yet decided how to reprimand these countries.

- 'Frank' talks -

Meloni and Rubio met at her Palazzo Chigi office for almost 90 minutes, after talks with his counterpart, Foreign Minister Antonio Tajani.

Earlier this week, Meloni said pulling US troops out of Italy was "a decision that does not depend on me, and one that I personally do not agree with".

Her office said the talks with Rubio were "broad and constructive", but also "frank", covering bilateral relations, the Middle East, Libya and Ukraine.

"It was a frank dialogue between allies defending their own national interests, but both recognising the value of Western unity," the statement said.

"We both understand how important the transatlantic relationship is, but we also both understand how necessary it is for each side to defend its national interests," the statement added.

In an interview with an Italian newspaper last month, Trump said he was "shocked" at Meloni's attitude, saying: "I thought she had courage, but I was wrong."

- 'Share points of view' -

Rubio, a devout Catholic, said on Friday that his meeting the previous day with Pope Leo XIV, the first US pope, was "very good".

Trump last month accused the head of the world's 1.4 billion Catholics of being "weak on crime and terrible for foreign policy" after Leo made critical comments about the Middle East war.

Rubio said they discussed topics of common interest, including religious freedom, the threat posed by Iran, and the role of the Catholic Church in delivering American humanitarian aid to Cuba.

"It's important to share our points of view and an explanation and an understanding of where we're coming from. And I thought it was very positive," he said.

Rubio, who also met with Vatican Secretary of State Pietro Parolin, said: "I updated them on the situation with Iran, expressed our point of view about why this is important, and the danger that Iran poses to the world."

The discussions between the pope, Parolin and Rubio addressed "the need to work tirelessly for peace", according to the Vatican.

Asked whether Trump would call Leo, Rubio said: "Maybe. I don't know, I mean, it could happen."

At the Italian foreign ministry, Tajani and other officials presented Rubio with documents tracing the US diplomat's Italian origins.

"It's a true honour and a very special moment to receive all of this information," Rubio said, adding that he was going to learn Italian.

The Cuban-American, who speaks fluent Spanish, said: "The next time I'm back... I'll give a speech 'in Italiano'."