Iran's President Hassan Rouhani rejected any hopes of rewriting a nuclear deal with world powers Wednesday, while his French counterpart vowed to US lawmakers that everything would be done to stop Iran ever acquiring atomic weapons.

In their different ways, Europe, Iran and Russia are all seeking to convince US President Donald Trump not to abandon the landmark 2015 deal that curbed Iran's nuclear programme in exchange for sanctions relief.

Trump has called the existing accord "insane" and "ridiculous", in part because its restrictions start to expire in 2025.

But addressing the US Congress on the last of his pomp-filled, three-day visit to Washington, French President Emmanuel Macron said: "Iran shall never possess any nuclear weapons. Not now. Not in five years. Not in 10 years. Never."

He has proposed an additional deal that extends Iran's nuclear restrictions while also curbing its ballistic missile programme and support for militias in the Middle East.

In Iran, Rouhani took a more aggressive tack, reiterating that the Islamic republic would not accept any changes to the deal.

"We have an agreement called the JCPOA," said Rouhani in a fiery speech, using its technical name.

"It will either last or not. If the JCPOA stays, it stays in full."

From Europe, a host of voices have urged Trump to build on the deal, rather than tear it up.

"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," said EU diplomatic chief Federica Mogherini as she arrived for a Syria donor conference in Brussels.

Moscow also reiterated its support, with Kremlin spokesman Dmitry Peskov telling reporters: "We believe that no alternative exists so far" to the nuclear deal, and demanding that Iran be involved in any further discussions.

The next to press the point will be German Chancellor Angela Merkel when she visits Washington on Friday.

"The biggest priority is maintaining the existing nuclear accord. It was negotiated by seven countries and the EU and can't be renegotiated by one or other party," said German foreign spokesman Rainer Breul.

"President Macron… spoke of the need for supplementary agreements. The question is whether, and in what circumstances, Iran would be ready to enter into such a process," he added.

– 'A high-rise builder' –

Rouhani's speech was particularly fiery, ridiculing Trump as "a trader, a businessman, a high-rise builder, how can he judge about global issues?

"You have no expertise in politics, nor in law, nor in international accords," he said.

Iran has the support of all other parties to the accord, who say it is working and Tehran has stuck to its commitments.

Macron, who has built a warm rapport with Trump, has positioned himself as an emissary for European officials seeking a compromise that allows the mercurial US president to claim a public victory, while keeping the deal intact.

But for now, even Macron had to admit he could not read whether Trump would simply walk away from the existing deal when the renewal deadline arrives on May 12.

True to his background in reality TV, Trump has kept everyone guessing.

"This is a deal with decayed foundations," he said. "We're going to see what happens on the 12th."

Iran has warned it will ramp up enrichment if Trump scraps 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," Trump said.

– Oliver Stone 'shocked' –

The French president's lavish visit to Washington did not impress one long-time critic of US foreign policy, Hollywood director Oliver Stone, who happened to be in Tehran for a film festival on Wednesday.

Stone told AFP he was "particularly shocked" to see Macron standing alongside Trump, calling for a revision of the nuclear deal.

"To see this young man, who doesn't have much of a sense of history or memory of the great traditions of France… was very depressing," he said.

Hawkish American officials take the opposite view — that the Europeans, particularly Germany, oppose a tougher stance against Iran to safeguard their business interests there.

That is sharply rejected by European officials, who are increasingly frustrated at spending time handling Trump's complaints rather than focusing on the deepening crises of the Middle East.

The disagreement threatens to plunge transatlantic relations to their lowest point since the Iraq war, despite the lavish welcome Trump put on for Macron's state visit.

And meanwhile, Western countries have found themselves increasingly sidelined from issues such as the Syrian war — underlined again on Wednesday when Iran, Russia and Turkey announced a new summit to discuss the conflict this Saturday in Moscow.

The 2015 Iran nuclear deal
Paris (AFP) April 25, 2018 –

In a hard-won deal struck in 2015, Iran agreed to freeze its nuclear programme in return for the lifting of punishing international sanctions.

It was a breakthrough that ended a 12-year standoff between Iran and the West, concerned that Tehran was developing a nuclear bomb.

But amid deep transatlantic divisions it now hangs in the balance and on Tuesday US President Donald Trump and his French counterpart Emmanuel Macron called for a "new" deal.

Here is some background:

– 21 months of talks –

Negotiations start in June 2013 between Iran and the five permanent members of the UN Security Council — Britain, China, France, Russia and the United States — plus Germany.

By November they reach an interim deal that is finalised in April 2015 and signed on July 14 that year.

The UN Security Council adopts the deal on July 20, 2015 and it comes into force on January 16, 2016.

– Main points –

The accord brings to a minimum of one year, for at least 10 years, the "breakout time" that Iran needs to produce enough fissile material to make an atom bomb.

Among other points, Tehran agrees to slash the number of centrifuges, which can enrich uranium for nuclear fuel as well as for nuclear weapons, from more than 19,000 to 5,060, maintaining this level for 10 years.

The deal limits all enrichment to only one facility.

It also stipulates that Iran's pre-deal stockpile of 12 tonnes of low-enriched uranium — enough for several nuclear weapons if further enriched — must be reduced to 300 kilogrammes (660 pounds) for 15 years.

Only enrichment to low purities is allowed, also for 15 years.

– Controls –

The International Atomic Energy Agency (IAEA) is charged with regular inspections of facilities such as uranium mines and centrifuge workshops for up to 25 years.

UN inspectors have certified Iran's compliance with the deal nine times, most recently in November.

– Sanctions eased –

The accord paved the way for a partial lifting of international sanctions on Iran, opening the door for foreign investors with French energy giant Total and carmakers PSA and Renault quick to strike deals.

UN embargoes on the sale of conventional arms and on ballistic missiles to Iran have however been maintained up to 2020 and 2023 respectively.

– May 12 ultimatum –

In October 2017, Trump refuses to certify that Iran is respecting its commitments on the agreement but he does not re-impose sanctions or abandon the deal itself.

Trump again waives sanctions in January but he demands European partners "fix the terrible flaws" in the accord before May 12, the next decision deadline for Trump on the deal.