The European Union cannot provide legal and economic guarantees to Iran but is serious about seeking a way to keep investment flowing and will come forward with measures in the next few weeks, the EU's top diplomat has said.

After a meeting of the foreign ministers of Iran, Britain, France and Germany, EU foreign policy chief Federica Mogherini said the group had tasked experts to protect European business in Iran.

The group will meet again in Vienna next week at the level of deputy foreign ministers.

"We are working on finding a practical solution," Ms Mogherini told a news conference.

"We are talking about solutions to keep the deal alive," she said, adding that measures would seek to allow Iran to keep exporting oil and for European banks to operate.

Europe and Iran have sought a united front to save the nuclear deal that US President Donald Trump abandoned last week.

The UK has warned against regime change, while the Iranian delegation expressed hope it could keep the economic benefits of the accord.

Iran's Foreign Minister Mohammad Javad Zarif arrived in Brussels ahead of a meeting with his British, French and German counterparts later, holding "good and constructive" talks with Ms Mogherini.

"We are on the right path to move forward ... Whatever (is) decided should preserve and guarantee Iran's rights ... Our talks will continue in the next two weeks," he said, referring to Britain, France and Germany.

Many European diplomats doubt privately that the 2015 accord between Iran and six world powers can survive the reimposition of US sanctions ordered by Mr Trump, but the European powers will say that they stick by the terms of the 2015 pact giving Iran sanctions relief in return for an end to its nuclear ambitions.

Iranian President Hassan Rouhani asked the European Union to stand against the US’s "illegal and illogical" actions, saying that Tehran could stay in the accord only if it fully benefits from it.

The accord lifted international sanctions on Iran in 2016 in return for Tehran shutting down its capacity, under strict surveillance by the UN nuclear watchdog, to stockpile enriched uranium for a possible atomic bomb.

Mr Trump has denounced the accord, completed under his predecessor Barack Obama, as a "horrible, one-sided deal that should have never, ever been made" as it did not cover Iran's ballistic missile programme or its role in Middle East conflicts.

The deal's proponents say it is crucial to forestalling a nuclear Iran and preventing wider war in the Middle East.

Ms Mogherini, who as the EU's top diplomat chaired the final stretch of 12 years of negotiations to clinch the Iran accord in July 2015 in Vienna, said: "We will all save it together."

Mr Zarif said talks would go on for the next two weeks and EU diplomats said they needed some time to understand the US position.

"One of the questions that we need to ask the Americans is whether their final objective is to make the Iranians yield on its nuclear programme or to get rid of the regime," said a senior French official, acknowledging that Paris was concerned by the ideological shift in Washington since John Bolton was appointed US National Security Advisor.

Those comments were echoed by British Foreign Secretary Boris Johnson who told parliament that regime change in Iran was not a policy Britain should pursue.

French diplomats said they also wanted to assess Iran's intentions in sticking to the deal, but also to see how open it was to serious talks on other Western concerns.

French President Emmanuel Macron has said he wants to secure a wider deal after the US withdrawal with four pillars: limiting Iran's nuclear program in the short and the long term, restraining its ballistic missile program and curbing what the West views as its destabilising behaviour in Syria, Yemen, Iraq and Lebanon.