The appeal from two of the signatories to the landmark 2015 accord came as French President Emmanuel Macron was on a state visit, received with much pomp by US President Donald Trump.

Trump Tuesday laid transatlantic divisions bare, pillorying a three-year old agreement designed to curb Iran's nuclear program.

The US leader described it as "insane" and "ridiculous," despite European pleas for him not to walk away.

Instead, Trump eyed a broader "deal" that would also limit Iran's ballistic missile program and support for militant groups across the Middle East.

Macron said the agreement should impose tougher terms on Iran including a settlement in Syria, where it backs President Bashar al Assad.

Macron was to address a joint session of the US Congress on Wednesday, with more freedom to speak his mind on world issues after his talks with Trump.

Iran's President Hassan Rouhani on Wednesday questioned the legitimacy of these demands for a fresh nuclear agreement with Tehran.

"Together with a leader of a European country they say: 'We want to decide on an agreement reached by seven parties'. What for? With what right?" Rouhani said in a speech.

EU diplomatic chief Federica Mogherini insisted Wednesday that the current Iran nuclear deal was working and "needs to be preserved".

"On what can happen in the future we'll see in the future, but there is one deal existing, it's working, it needs to be preserved," the former Italian foreign minister said as she arrived for a donor conference on Syria in Brussels.

In Moscow, the Kremlin said there can be 'no alternative' to the current deal with Iran.

"We believe that no alternative exists so far," President Vladimir Putin's spokesman Dmitry Peskov told reporters, adding that Iran's position on the subject was paramount.

Trump said earlier that "they should have made a deal that covered Yemen, that covered Syria. No matter where you go in the Middle East, you see the fingerprints of Iran behind problems."

Looming decision

Macron said after meeting Trump that he did not know whether the US president would walk away from the nuclear deal when a May 12 decision deadline comes up. 

Putting on a brave face, he said he wished "for now to work on a new deal with Iran" of which the nuclear accord could be one part.

Speaking to US lawmakers on Wednesday in English, Macron will address what he thinks are historically warm Franco-US ties now challenged by differences over economic inequality, climate change, how to tackle terrorism, threats to democracy and the rise of nationalism, according to sources in Macron's entourage.

Macron, completing a three-day state visit, was also scheduled to visit the Library of Congress and be received by House of Representatives speaker Paul Ryan.

On Tuesday neither Trump nor Macron indicated whether Iran would get something in return for concessions on its ballistic programs, activities in the Middle East or extending nuclear controls beyond 2025.

Trump -- true to his background in reality TV  -- teased his looming decision.

"This is a deal with decayed foundations. It's a bad deal, it's a bad structure. It's falling down," the US leader said. "We're going to see what happens on the 12th."

Trump's European allies have repeatedly tried to persuade him not to abandon the 2015 deal, which gave Iran massive sanctions relief and the guarantee of a civilian nuclear program in return for limiting enrichment that could produce weapons grade fuel.

German Chancellor Angela Merkel will renew those calls when she visits Washington on Friday.

Iran, meanwhile, has warned it will ramp up enrichment if Trump walks away from the accord, prompting a blunt warning from the US leader.

"They're not going to be restarting anything. If they restart it, they're going to have big problems, bigger than they ever had before. And you can mark it down," he said.

Simmering tensions

For months American and European officials have been working behind the scenes trying to find a compromise on Iran that allows the mercurial US president to claim a public victory, while keeping the deal intact.

More hawkish American officials accuse Europeans -- particularly Germany -- of putting business interests ahead of security, and of opposing a tougher stance against Iran to safeguard investments in the Islamic Republic.

That charge is sharply rejected by European officials, who are increasingly frustrated at spending time dealing with Trump's complaints rather than tackling Iran's behavior.

The disagreement threatens to plunge transatlantic relations to their lowest point since the Iraq War.

Trump's comments on Iran contrast markedly with the exuberant welcome he gave the French leader.

Tuesday morning both men waxed lyrical about shared heroes of yore -- from the Marquis de Lafayette to Alexis de Tocqueville -- as they listened to strains of "La Marseillaise" and "The Star-Spangled Banner."

Later, the Trumps rolled out the red carpet for the Macrons once more at a lavish state dinner -- of which the US first lady was said to have fine-tuned every last detail, from gold tableware to white floral centerpieces, for star guests including Apple CEO Tim Cook and media mogul Rupert Murdoch.

Melania Trump dazzled in a sequined Chanel dress of black Chantilly lace, while Brigitte Macron stepped out in a long-sleeved creamy white gown with gold lattice detailing by Louis Vuitton.

The key question is whether Macron can translate their privileged relationship into concrete results -- as he also pushes for a permanent exemption for Europe from US steel and aluminium tariffs.

Earlier in the Oval Office, Trump offered a striking -- and slightly awkward -- sign of their much-vaunted intimacy.

"We have a very special relationship, in fact I'll get that little piece of dandruff off," Trump said, swiping something off Macron's jacket. "We have to make him perfect -- he is perfect."