President Joe Biden admitted Friday to "clumsy" dealings with France following a bitter row over a submarine deal, at the start of a European trip designed to show off US leadership -- and mend some fences.

The president deployed a charm offensive with French President Emmanuel Macron in their first meeting since Paris erupted last month over news of a new US-UK-Australia submarine deal that left its own multibillion deal with Canberra in tatters.

"What we did was clumsy, it was not done with a lot of grace," Biden told Macron in Rome, where both are gathered for this weekend's leaders summit, adding that "we have no better ally than France".

Biden will head first to the Vatican, for what is expected to be a largely good-humoured sit-down with Pope Francis
Biden will head first to the Vatican, for what is expected to be a largely good-humoured sit-down with Pope Francis GETTY IMAGES NORTH AMERICA / CHIP SOMODEVILLA

It was the clearest sign of contrition from the US since the start of a diplomatic row that saw France recall its ambassadors from Washington and Canberra.

And it marked a sharp contrast to the past four years of Donald Trump's bruising diplomacy, setting a tone for a trip that includes landmark UN climate talks next week.

Macron welcomed efforts by the US to defuse the crisis, saying: "What really matters now is what we will do together in the coming weeks, the coming months, the coming years."

Biden said the meeting, part of which was with his wife Jill, left, had been "an honour"
Biden said the meeting, part of which was with his wife Jill, left, had been "an honour" VATICAN MEDIA / Handout

A joint communique issued after the meeting said the US had committed "additional assets" to France's counter-terrorism efforts in the Sahel.

Biden will head first to the Vatican, for what is expected to be a largely good-humoured sit-down with Pope Francis
Biden will head first to the Vatican, for what is expected to be a largely good-humoured sit-down with Pope Francis GETTY IMAGES NORTH AMERICA / CHIP SOMODEVILLA

Earlier, Biden had an apparently easier meeting with Pope Francis at the Vatican, from which he emerged with the message that he was a "good Catholic".

In a meeting that lasted more than an hour -- longer than his two predecessors were given -- the two men stuck to subjects on which they agree and sidestepped the controversial topic of abortion.

The US president thanked the pope in a tweet afterwards "for his advocacy for the world's poor and those suffering from hunger, conflict, and persecution, and lauded his leadership in fighting the climate crisis and ending the pandemic".

Biden turned on the charm meeting his French counterpart after Paris' furious reaction to a US-UK-Australia submarine deal that left a French deal with Canberra in tatters
Biden turned on the charm meeting his French counterpart after Paris' furious reaction to a US-UK-Australia submarine deal that left a French deal with Canberra in tatters AFP / Ludovic MARIN

The talks were behind closed doors but footage released by the Holy See showed a good-humoured gathering full of smiles, with the president at points visibly moved, and elsewhere telling the pope, "God love ya".

He called the pontiff "the most significant warrior of peace I have ever met", as he gave him a presidential coin recalling the regiment in which his son Beau Biden, who died from cancer in 2015, had served.

"I know my son would want me to give this to you," Biden said.

The president, who is open about his faith and how it gives him strength, had already met Francis three times before but this was their first tete-a-tete since he entered the White House.

"We just talked about the fact that he was happy I was a good Catholic and I should keep receiving Communion," Biden told journalists after the meeting, which his wife Jill attended part of.

Biden supports the right to choose, while Francis, 84, has called terminating pregnancies "murder".

The pontiff has nonetheless distanced himself from a push by conservative US bishops to deny communion to politicians supportive of abortion rights -- which would include Biden.

The Vatican said they discussed climate change, the pandemic and the issue of refugees and migrants, as well as "the protection of human rights, including freedom of religion and conscience".

Both Biden and the pope have been outspoken on the need to tackle global warming, and Biden will attend the COP26 climate talks in Glasgow starting Monday, with a message that America is once again ready to lead.

Shortly before leaving Washington, Biden unveiled a "historic" blueprint for remaking America's economy, a $1.75 trillion Build Back Better social welfare package on which he has staked his domestic legacy.

He had hoped to secure the deal before leaving, but it has been dogged by weeks of internal party feuding -- it remains to be seen if the revamped deal will gain the support of lawmakers.Biden said his new plan includes "the most significant investment to deal with the climate crisis ever" -- $550 billion to cut greenhouse gas emissions.