US President Joe Biden went on a charm offensive with European allies at the G20 summit in Rome, unveiling a steel deal and promising to mend ties strained to breaking point by Donald Trump.

It was a fresh start in an ancient setting: those aboard the gigantic presidential convoy -- over 80 vehicles at times -- glimpsed the Vatican, Colosseum and Roman Forum as they were whisked around the Eternal City to high-end meetings this weekend.

Sunday saw Biden hold a news conference with European Commission President Ursula von der Leyen to celebrate "a new era" and a "milestone" in the transatlantic relationship.

They announced the end of a trade dispute on steel which dated back to the Trump presidency and promised to work together to fight climate change and resist Chinese competition.

Biden said the "major breakthrough" was "testament to the power of our strong partnership".

"We have restored trust and communication," von der Leyen responded, after Trump's "America First" policy alienated stalwart US allies across Europe and beyond.

Biden said the summit demonstrated the "power of America showing up" and working with its allies to forge progress on key issues.

"We got significant support here, significant support. The US of A is the most critical part of this entire agenda, and we did it," he said.

'Tangible progress'

The talks went ahead without the leader of China, the world's largest emitter. President Xi Jinping instead appeared via video link, as did Vladimir Putin of Russia.

Biden said he was disappointed that Russia and China "basically didn't show up in terms of any commitments to deal with climate change".

"There's a reason why people should be disappointed in that. I found it disappointing myself," he said.

Nevertheless The US president said that the leaders who met in Rome had made "tangible progress" on key issues, from climate change to the coronavirus pandemic and the economy, ahead of the COP26 climate talks that opened on Sunday in Glasgow.

The US president on Friday sat down for talks with Italian Prime Minister Mario Draghi and French President Emmanuel Macron.

The Democrat adopted an apologetic tone with Macron following a diplomatic crisis over a submarine deal, visiting the French embassy near the Vatican and proclaiming his "strong affection" for France.

Biden acknowledged the US had been "clumsy" in the affair, which saw it sign a deal with Australia for nuclear submarine technology at the expense of France.

Macron welcomed the efforts by the US to defuse the crisis, saying: "We clarified what we need to clarify."

Nuclear umbrella

With Xi and Putin absent from Rome, Biden had the geopolitical limelight largely to himself.

He also had a four-way session with Macron, Germany's Chancellor Angela Merkel and British Prime Minister Boris Johnson to harmonise positions on the Iranian nuclear crisis, and met separately with Merkel's likely successor, Olaf Scholz.

There were fences to mend over America's chaotic withdrawal from Afghanistan, after Biden insisted on a total pull-out of US forces -- requiring European allies to follow suit.

A senior US official flagged up the chance to "make as much progress as possible while there is a (US) president in office who is deeply committed to the transatlantic relationship".

The same official also downplayed any differences of opinion on China.

Biden has rejected talk of a new Cold War with Beijing but has adopted a hard line that is not unanimously supported in Europe.

The official insisted he had seen "strong convergence" with the Europeans "on the nature of the challenge".

Washington has also promised to "consult closely with its allies" on a review now under way of the US nuclear posture.

This involves the world's leading power formalising the circumstances under which it might use nuclear weapons, particularly if its allies were attacked.

Britain, France and Germany, along with Australia and Japan, are concerned that the US could shrink its nuclear umbrella, according to the Financial Times.

Biden and Macron issued a joint statement on Friday in which they pledged "close consultations” on nuclear issues.