Europe is scrambling to come up with ways to persuade Iran to stick with the landmark 2015 agreement despite US President Donald Trump ditching it a week ago.

Tehran has warned it is prepared to resume "industrial-scale" uranium enrichment "without any restrictions" unless Europe can provide solid guarantees that it can maintain the economic benefits it gained from the nuclear agreement despite Washington reimposing sanctions.

Iranian Foreign Minister Mohammad Javad Zarif met EU foreign policy chief Federica Mogherini and his counterparts from Britain, France and Germany -- the three European signatories to pact -- in Brussels on the last leg of a whirlwind diplomatic tour that also took in Russia and China, the two other signatory nations.

Mogherini said EU experts were aiming to come up with concrete proposals in the coming weeks on nine key issues including ensuring Iran could sell its oil and gas products and have access to international finance.

"I believe it's a good start. We're not there, we're beginning the process," Zarif told reporters after the talks.

But he warned that Tehran expected to see progress towards the guarantees it wants "within the next few weeks".

Mogherini acknowledged the enormous challenge of finding a way around US sanctions punishing businesses trading with Iran, which apply all around the world.

"We know it's a difficult task but we are detemined to do it and we have started to work to put in place measures that help ensure this happens," she told reporters.

EU experts have already started work on measures to get round US sanctions on Iran, Mogherini said.

Their efforts focus on nine key areas including maintaining economic ties with Iran, continuing Iran's ability to sell oil and gas products and protecting EU companies doing business in Iran.

The EU is also looking at how it could develop special financing vehicles for doing business with Iran.

- 'Respect the deal' -

The European Union insists the deal is working, pointing to repeated UN inspections verifying the Islamic republic's compliance with its side of the bargain.

EU leaders aim to show a united front on preserving the Iran deal when they meet for a pre-summit dinner in Sofia on Wednesday, European Council President Donald Tusk said.

"I would like our debate to reconfirm without any doubt that as long as Iran respects the provisions of the deal, the EU will also respect it," Tusk said in a letter to the leaders on the eve of the summit.

European firms, especially those from France and Germany, rushed to invest in Iran following the 2015 accord, under which Tehran agreed to freeze its nuclear programme in return for an end to punishing international sanctions.

German exports to Iran totalled nearly 3.0 billion euros in 2017, while French exports soared from 562 million euros in 2015 to 1.5 billion in 2017 and oil giant Total has pledged to invest some $5 billion in the South Pars gas field.

- 'No illusion' -

When he quit the deal last week, US President Donald Trump gave businesses a maximum of six months to wind up operations in Iran or face swingeing penalties under American sanctions.

"We have to be realistic about the electrified rail, the live wire of American extra-territoriality and how that can serve as a deterrent to business," said British Foreign Secretary Boris Johnson.

French President Emmanuel Macron held phone talks with his Russian counterpart Vladimir Putin on Tuesday, and the Kremlin said they had "confirmed Russia and France's commitment to make the deal work".

Washington has long complained that the nuclear deal does nothing to stop Iran's ballistic missile programme or its interference in conflicts across the Middle East from Syria to Yemen.

On Sunday, US Secretary of State Mike Pompeo said Washington still wants to work with Europe to counter Iran's "malign behaviour" and was working hard to thrash out a more wide-ranging deal with its European partners.

But Mogherini cast doubt on the idea on Tuesday, saying the deal "doesn't need to be changed, modified or have any addition".

"We're not talking about annexes or modifications of the agreement at all, we're talking about setting up concrete measures" to preserve the deal, Mogherini said.

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